Getting Email Delivered - the ISIPP SuretyMail Blog

What Those SMTP Error Codes Mean and Why You Should Care

Summary: Anybody who sends email has seen them, in one form or another - those SMTP error codes, often returned in bounced email, such as "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site." These are often in response to SMTP commands that have 'gone wrong' between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. But what exactly do they mean? And why should you care? ('SMTP' stands for Simple Mail Transfer Protocol.) First, here is a list of SMTP messages and error messages, and what each message is supposed to say and mean

Anybody who sends email has seen them, in one form or another – those SMTP error codes, often returned in bounced email, such as “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site.” These are often in response to SMTP commands that have ‘gone wrong’ between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. But what exactly do they mean? And why should you care? (‘SMTP’ stands for Simple Mail Transfer Protocol.)

First, here is a list of SMTP codes and error codes, and what each message is supposed to say and mean:

211 – A system status message.
214 – A help message for a human reader follows.
220 – SMTP Service ready.
221 – Service closing.
250 – Requested action taken and completed.
251 – The recipient is not local to the server, but the server will accept and forward the message.
252 – The recipient cannot be VRFYed, but the server accepts the message and attempts delivery.

354 – Start message input and end with .. This indicates that the server is ready to accept the message itself

421 – The service is not available and the connection will be closed.
450 – The requested command failed because the user’s mailbox was unavailable (such as being full). Try again later.
451 – The command has been aborted due to a server error. (on their side)
452 – The command has been aborted because the server has insufficient system storage.

Unfortunately, many receiving systems seem to mix and match these error messages, rather than adhering to the prescribed code messages.

The good news is that the SMTP error messages you need to worry about as an email sender are really primarily limited to just a few – those in the 5XX range – and of those, the ones that you really need to worry about are the ones in the 55X range – i.e. 550-559. Of these, by far the most common one – the one that as a sender you will see most often and to which you must pay immediate attention – is the 550 SMTP error code, which can range from “user not found” to “mailbox unavailable” to any number of other similar variations, but which for you, dear mail sender, should almost always be read to mean “remove this email address from your mailing list immediately.”

(On the other hand, your mail server administrator should care deeply about many more of these codes, however we assume that if they are administering your outbound mail server, they are already familiar with them.)

So, back to that 550 error code. Almost always, when you get an email bounced back (or inserted in your mail log!) with a 550 error code, it means that the receiving system could not deliver your email to the user to whom it was addressed because the mailbox is unavailable. Almost always, this means that the inbox either no longer exists, or that it never existed!

Why would you have an email address on your mailing list that has never existed? There could be a lot of reasons, including that someone entered it wrong, or that someone intentionally entered a fake email address into your system (such as when you require a user to divulge an email address in order to receive a download, etc.).

Regardless of how it ended up on your mailing list, if you send a mailing to it, it tells the receiving system one sure thing: that you don’t confirm email addresses before adding them to your mailing lists.

And, if you send email to that same non-existent email address after receiving the 550 message that the mailbox doesn’t exist, it tells that receiving system that not only don’t you confirm email addresses, but that you don’t care very much about list hygiene – i.e. that you don’t maintain your mailing lists according to best practices, either.

And, having determined this, very soon those receiving systems, including ISPs, will simply stop delivering your email to the inbox – first diverting it to the junk folder, then not delivering it at all, and then perhaps even blacklisting all of your email.

Now, it’s possible that a mailbox will be unavailable because a user has let their inbox get full. And we often get asked whether, because of that possibility, it’s ok to not remove an email address which has returned a 550 error?

Think about this: in this day and age of nearly unlimited email storage (nearly all ISPs now offer multiple Gigs of email storage), just how inactive does a user have to be in order for their inbox to fill up? And, even if they are going to come back some day and clear out their inbox, do you think that they are really going to stop to read your email? Or will they delete it unopened and unread, creating even more problems for you?

The bottom line is: why hang on to an email address that belongs to a user who will never be a positive asset for you, and can only cause you trouble?

So the next time an email that you send bounces back, take a good look at the information in the bounce message, and take the appropriate action based on that message. And don’t forget to check your mail logs for bounce messages too!

P.S. If you are actually receiving bounce messages that include “mail from refused spam site” or a similar message, the odds are good that your mail already has been blacklisted, and you’ll need to deal with that immediately.

Article Name
What Those SMTP Error Codes Mean and Why You Should Care
Here's what those SMTP error codes such as "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site" mean.


    What about the 450 message? From what I know 4xx messages indicate a temporary failure, but isn’t a “full mailbox” also a temporary failure? A lot of MTA’s will immediately generate a bounce message for 550 but most will keep trying to deliver a message that fails with 450 for about a week ( or whatever the “queue life time” is for a certain server). If this considered harmful by the ISPs?
    Should we remove these email addresses ( that fail with a 450 error after trying for a week ) immediately from the list ( like we do with 550 codes ) or should we maybe send another mailing ?

    Some free email providers will deactivate an account if is inactive for X days. ( I think yahoo used to do this or still does ) . In this they would send a 450 message not a 550 message as the user can still reactivate his account by simply logging in.

  • Hi there,
    we have an unusual 550 problem, when we send out mail via our web site to a specific email address we get a 550 mail bounced straight back to us, however if we use the same email address vai outlook (using the same smtp server to send it) then the messages get through without a problem? Any suggestions??

    The bounce we get is (with actual email address an IP’s removed):
    Your message did not reach some or all of the intended recipients.

    Sent: 19 Jan 2010 20:07:38 +0000
    Subject: Password Reset

    The following recipient(s) could not be reached:
    Error Type: SMTP
    Remote server ( issued an error.
    hMailServer sent: .
    Remote server replied: 550

  • Hi Reuben

    To answer your question, you will need to tell us the specific 550 error, usually it will tell you what has happened. Also check whether your mail servers are not appearing on any spam blacklists.

  • Thanks for that exhaustive yet concise description which I have bookmarked. And while I did normally remove 550s I usually left the “over quota” guys in in the (as you explained quite vain) hope they might eventually clear up their mailbox and read the mails. No, you’re right, I probably wouldn’t either. But I wasn’t aware of the ramifications and that it would lower my “sender credit score” so to speak.

  • I’m having a 550 problem that I have not been able to get a solution on for more than 3 years. The error I get is:

    Your message did not reach some or all of the intended recipients.

    Subject: FW: The Clinical Device Newsletter
    Sent: 6.5.2010 08:12

    The following recipient(s) could not be reached:

    ‘’ on 6.5.2010 08:12
    550 Error: Lets just say we’re removing now.

    No one that I have been in contact with has any knowledge of this error. I have used Outlook 2003/2007, Eudora 8 and Thunderbird 2 and 3 and I allways get the same error sending HTML mail with pictures(these are newsletters). I use a POP account and my IT provider claims that this is not on his side, he has informed me that during this period he has changed mail-server hardware and software so that this should not be his fault. Can anyone help with this weird error??

    • The sending errors that you get from godaddy are the relays that they use, if you have a website from godaddy and it comes with free emails, these are limited to 1mb of their hard drive space allocated for emails, after sending out emails from
      Microsoft Outlook 2007 I started getting errors from 550-552….
      Called Godaddy, they didn’t know what to do, after spending 2 hours on the problem, found that I have reached my Out going limit, called Godaddy and had my email limit raise to unlimited, waiting for about 15 minutes for it to kick in…
      You can monitor your emails by logging into Godaddy and go to Launch in the emails section, you’ll see your limits and activity…
      I started sending out emails from Outlook 15 minutes after I had the limit changed, sent out about 1,100 in the same day, No problem sending or receiving….
      You do have to watch the size of your Image files that you use in the signature part though, you are limited on the size of files being used…

      Note: When sending a lot of emails, use an Antivirus that put their Logo into the Send and Receive emails on the bottom…There are 2 Companys that do This, the one that I use is Avast 2015, it checks your email before it sends and then puts their logo and time stamp at the bottom of the email, same thing when receiving emails… Email Servers like this, because it does there work for them, and the emails go through faster, plus it protects you and your Customers…….

    BTW. There is no problem sending text mail even HTML text mail to the same recipents.

  • Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

  • Same here. I am the host and I get this email bouncing back but it sends fine in Gmail.

  • Hey Dude …(assuming you are a Dude)

    I give this article an A+

    Excellent with very useful information

    Thanks GC

  • hey im having the same problem as TIN

    May 11, 2010 5:26
    Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

    recently ive been trying to contact someone as well an their email has worked jus fine for years and its like all of a sudden i cant get them on IM and i cant get them in email my mail bounces back and im not sure if maybe they just havent been on and it could be that their mailbox is just full. but this is the first time in years ive ever had a problem with this..please help!

  • I think it can also be said that just because you have a 55X error message, doesn’t necessarily mean your list isn’t clean or that you don’t check your list for hygiene.

    Consider the case where a mail sender hasn’t e-mailed the list in longer than 30 days — it is quite possible to receive a 55X range error message in that case, because people close their e-mail accounts, etc quite frequently.

    That said, I think it can be argued (and in my own experience) most ISP’s don’t just automatically assume that you don’t confirm your e-mails simply because they issued you a 55X response code.

    Now of course, if you receive the same 55X error code for the same *exact* e-mail address a 2nd or a 3rd time, then yes, it can be assumed you don’t sanitize your mailing list, but simply receiving a 55X error code one time, doesn’t necessarily indicate that you don’t sanitize your list.

    This is especially true with mail providers such as Yahoo! and Gmail (in my own experience).

  • hey im having the same problem as TIN

    Hi… I need some help with the 550 returned mail error.. the email address was active few hours ago (and for the last 8 years), and then all my email to the recipient was returned with the the 550 error… how do I resolve this?

  • Ramesh,

    Please bear in mind that 550 could also mean that the user’s mailbox is full.

    Different ISP’s respond differently in this regard, so if after a few tries (say 2-3 tries) you’re still not getting mail delivered, it’s probably safe to temporarily remove the subscriber from your list (try again in a subsequent mailing).

    Probably the mailing after your next one depending on the frequency of your mailings may be a good idea (for example, if you e-mail once per month, it’s probably safe to e-mail them in the month following your next mailing).

    If at that point you receive a 550 again, it’s probably safe to permanently remove the e-mail from your list for the following reasons:

    1) If the mailbox is full, it probably means the user is not actively reading/monitoring e-mail sent to that account in the first place.

    2) Something else could be wrong, but in any case, no point in mailing to a subscriber who is almost surely not receiving your messages anyway.

    Let me know how you get on. . .

  • We do electronic billing. Have just received this message:
    “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.

    After sending successfully for the past few weeks, this just occurred today. Am I reading correctly that the recipient is telling me that we are sending “spam?”

  • Richard,

    No probably not.

    The problem is on the recipient’s side, not yours:

    “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator”

    This means something is wrong on their end (assuming you pasted the error message in its entirety).

  • hi
    I have The problem to conect mailserver
    title eror is :
    SMTP -> FROM SERVER:220 ESMTP MDaemon 10.0.5; Thu, 24 Nov 2011 09:36:30 +0330
    SMTP -> FROM SERVER: Hello, pleased to meet you 250-ETRN 250-AUTH=LOGIN 250-AUTH LOGIN CRAM-MD5 250-8BITMIME 250 SIZE
    SMTP -> FROM SERVER:250 , Sender ok
    SMTP -> FROM SERVER:550 must check for new mail first
    SMTP -> ERROR: RCPT not accepted from server: 550 must check for new mail first
    SMTP Error: The following recipients failed: SMTP Error: The following recipients failed:

    SMTP server error: must check for new mail first

    plz help me .

  • Very good article. However I would disagree about hasty removal of the email that returns the 550 error message because in many corporate and government organisations who are using their own servers, limiting Storage Space for employees is their policy. Many emails now contain photos, graphics which are huge MBs. If one doesnt delete them regularly, then the Mail Box simply is Full. Suggest to wait for a few days and deliver your email again or if it is someone you know, just call up on a phone.

  • I am not sure if I agree with all that was said although I didn’t understand all that I read. However, the relevant parts are that… I just decided to begin using Outlook with the help of another Outlook user who has since left and with whom I usually contact by email.

    I am now unable to send email by Outlook which is where I received my first 550 error message. So I went to send an email through the original site and received the same error message.

    Please advise

    Now the parts I don’t agree with is about not ever checking email once the box is full (which happened to be a work email with very limited space).

    When do get a chance to check clean out the box then I do open each email and check for relevancy.

  • Thanks so much for the very useful information! I’m bookmarking it now…

    - KJ

  • Someone trying to send to us is getting the following bounce returned to them…

    There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.

    I cannot work out if this issue is at our end or theirs.

  • I have emailed my best friend for years from my work email. All of a sudden, when I tried to reply to an email from her it was bounced back with a 550 5.1.1 user unknown message. I tried composing a new email, but it was also bounced back. And the weird thing is that while she is in my address book, her email address does not “auto finish” when I type in the first letter of her address like it usually does. I have no problem emailing her using my gmail, it is only in Outlook. She knows of no changes on her end and hers is the only address that I have noticed being bounced back. What can I do?

  • I also am getting mail bounced back/rejected: Here is the error message. Technical details of permanent failure:
    Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Requested action not taken: mailbox unavailable (state 17).

    What does this mean? How can I resolve it……..NOW…lol Thanks

  • Hi,

    I am also annoyed with 55X errors messages.

    1st, when member register with our website, an auto-generated email goes to him/her to confirm and even those emails are not reaching to their mailbox. They click on resend again and again but they never get the email to confirm their email.

    Example messages are:

    1. 550 Recipient Rejected: Access Denied: blocked by prs
    2. 550 5.1.1 : Recipient address rejected: User unknown
    3. 554 delivery error:dd This user doesn’t have a account
    4. 550 5.7.0 Blocked
    5. 554 5.7.1 SPAM or UCE is not accepted here

    And when I check reputation of my IT through Return path, i find it 97. SPF record is pass.

    2nd, when I send emails to DOI members (those who have already confirmed email address), I again receive even more bounce backs with similar messages.

    What could be a solution here?

  • FYI, a full mailbox 550 can happen if a user is forwarding all their mail to another address and doesn’t realize they have a box filling up somewhere in cyberspace. That happened to me years ago until friends told me they had a problem with my address and I figured it out. You can’t simply assume they aren’t reading any of their email.

  • if you have 550 message error its alsa possible its because your smtp server is blacklisted by the recepient mail server (spam protection)

    Its what happened to me

  • This is VERY interesting for me thank you

  • Error sending email – 554 Transaction failed : Cannot send message due to possible abuse?


    My Outlook was working perfectly so far. But From today morning I am not able to send mails. I am getting error message as following…

    554 Transaction failed : Cannot send message due to possible abuse; please visit for more information

    Please help me to solve this. I have tried searching many sites. Everywhere they were explaining the error but no solution for it.


  • Ramyah,

    Your IP address is likely on a PBL somewhere, which is why Yahoo is rejecting your messages.

    Try — if your IP appears on their list, this is the likely cause.

    You’ll need to request removal of your IP, but I have to forewarn you, SpamHaus is very specific about their removal policy, and if your IP is dynamic (assigned by your ISP and changes frequently), odds are you will not be able to get it removed.

    However, if your IP is dynamic, simply rebooting your modem will likely cause you to get a new IP with which you may be able to send mail again to Yahoo.

  • thanks for the great summary-
    I have a 550 problem-
    here is the error message I receive:

    [Return Code 550] sid: f0vn1m00n5A5ucV01 :: Requested action not taken: mailbox unavailable

    howeer I have sent the email to 20+ people– so I do not know who the message has bounced from– and the attached/returned email is the original email to all 23 folks…
    I would guess that sid number is something meaningful — however I would like to know whos mailbox is unavailable…

    thanks for your help with this!!

  • JH,

    The 550 error shown was produced within a session. Depending on how you’re getting that error result, there might be additional information on the message error itself that tells you during which SMTP conversation it happened.

    As a way of example, the error message might include phrases such as “… while talking to SMTP server blah…” (here “blah” would be the remote mail server that returned the error and could be a hint as to which email address caused the error).

    If that does not help enough, you can try mailing the candidate addresses in smaller groups or even one by one and seeing which one returns the error message.

    Hope this helps.

  • I have been troubleshooting the following email bounce error code for the last few days:

    [Return Code 554] sid: j2971m00E2mNsTW01 :: Denied

    As it turns out, this problem relates to the dynamic IP address that was recently assigned to my Comcast cable modem/router. The bounces had nothing to do with my email host or domain name. First you have to access your router. My LAN address for the router is and then you type in the username and password. Go to the “Gateway Summary” tab on the left and then click the “Network” tab at the top. Copy the “WAN DHCP IP Address” that your router is using and check it against If the dynamic IP address is on a spam list, then this is likely the source of the bounces. Call Comcast and get them to change your dynamic IP address or pay for a static IP address ($15 per month). Good luck with this last part. They have the worst customer service of all time.

  • JH,

    Well the problem is actually asymptomatic of a larger issue.

    Comcast is not to blame here for assigning you a dynamic IP, as that is common practice among internet service providers (unless you pay for a static IP or upgrade to a business account in some cases).

    I have a dynamic IP address as well with my provider here in Florida (Brighthouse Networks [formally Time Warner Cable]).

    The main thing you have to consider is that spamhaus by default lists most dynamic IP ranges in their PBL (Policy Block List), which includes most ISP IP address ranges.

    And the purported reason for them establishing this PBL in the first place is, according to their website:

    “The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.”

    So while I hate when ISP’s and MSP’s arbitrarily take action that affects e-mail delivery for legitimate e-mail senders/servers, in this case, I can totally agree with the Spamhaus PBL and at the same time help you understand why your message was blocked.

    Consider that no dynamic address should be “delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer’s use.” (per Spamhaus’ PBL explanation, which I am in total agreement with)

    Back in the early days of the internet users were able to send e-mail directly from mail transfer applications on their machines, which in the beginning served a unique purpose, since back then most people didn’t have access to a server with which to send SMTP authenticated e-mail messages to others, so it was either use your personal e-mail, or use a mail transfer program.

    However, ultimately, like with other simplified methods of transmitting e-mails, this method of sending mail was soon exploited by those sending Unsolicited Commercial E-mail messages (UCE, or commonly referred to nowadays as SPAM).

    That left ISP’s with a unique conundrum. . .block port 25 to prevent the spread of spam e-mail from their users, or allow the problem to continue unabated.

    Most ISP’s obviously went the the prior solution, since it stopped a large amount of spam (for the most part, since PostMaster for example was configurable, and could be configured to use different ports), and eventually led to the ISP’s simply blocking unauthenticated e-mail messages from being transmitted at all.

    As the internet evolved, the price of domains and hosting dropped to levels that made it basically affordable for almost anyone to setup a hosting account or buy a domain and have the registrar host their e-mail.

    This made it possible for common everyday users to send mail using proper SMTP authentication, but more importantly, ISP’s further locked down their networks and forced users to authenticate using their own servers, or to authenticate using their networks directly (they would provide you with an SMTP address like or something similar).

    Now of course, this is a seriously minimized explanation of the overall transpiring of things as they played out, but the point is. . .Comcast is not to blame here, it is spammers instead who are to blame. ;-)

    As for the reason you got an error, it’s because you sent mail directly from your machine, in an unauthenticated format, which modern day e-mail service providers (Gmail, Hotmail, Yahoo, etc), as well as modern day e-mail servers are able to easily detect and block.

    Some of the mail service providers use Spamhaus’ database to determine if they should accept or block an e-mail message, as well as many independent mail servers on the internet (mine does for example).

    If the mail originates directly from an IP address (which would have no rDNS, and no resulting mail server that can receive an error message), it will get blocked in most cases, just like yours did.

    What you should do is ask Comcast how to send e-mail because your messages aren’t being sent using SMTP authentication (where your application is actually providing a username/password to the server that it is using to send the e-mail message).

    They will in turn give you an address you can use to send outbound e-mail.

    That, or if you have your own domain/hosting, you can simply configure your mail client (Outlook, Thunderbird, etc) to connect to your server, and authenticate mail from you.

    Your messages will more than likely be delivered if you go that route. ;-)

  • I send out an email, and get back a diagnostic that says “553 Sender is on user deny list (Mode:normal). Help, as it is quite important that I can emai this individual. The problem just started a while back. Before, there was no problem

  • Paul:

    The error message you’re receiving means you (“sender”) are on a the recipient’s (“user”), deny list — this list is maintained by the user in most cases (meaning the user put you on this list because they didn’t want to receive your messages).

    You should probably e-mail the user from a different address, or use a different method of contacting the user, such as a phone call for example.

    If the user has you on a deny list and it wasn’t an accidental placement, the user likely doesn’t want to receive your messages any longer.

    Sometimes users do this, because they don’t want to say: “No” to an upsell your pushing for example, or because they don’t want to be rude in an e-mail to you.

    The reasons are endless, but based on your message it sounds like this may very well be the case.

  • I’ve been getting the 550 error as follows. Failed to send email
    Error occurred on email server.-550- requested action not taken: mailbox unavailable failure sending mail try again later…. I have tried sending multiple messages to multiple address with same results. A adress via Android email app… if I go on the mail .com website the same message to same adresses send just fine.. even more strange is that sometimes one email will randomly send although most dont. This Is quite frustrating and nothing in above comments says anything like it. Any thoughts? I’m thinking something in outgoing server settings but all commands are so buried on the android every option in that settings page does nothing. Also deleting account from app then trying it new will work for one email sometimes then same error occurs. DNs problem? How do I get into DNs settings via the app any solutions? Is it simple and I’m just retarted?

  • Hi,

    Unfortunately your error message isn’t an Android error message, but instead an error being returned by the e-mail server you’re trying to use to send. You should contact your provider ( to find out why they aren’t allowing you to send mail using your Android device.

    If you suspect the issue to be DNS related — that then lends the question are you connected to your mobile network or are you using a Wi-Fi connection??

    On a mobile network, your provider would control your DNS — using Wi-Fi would mean that you control the DNS (if you’re connected to your own Wi-Fi network, otherwise it would be the person who controls the Wi-Fi network that controls the DNS in question).

  • I’ve had the same problem with both wifi and mobile network Which Probly means it’s the email server. What’s a good email to sign up for that dosent spy excessively or ask for a bunch of personal information to sign up?

  • I’m having an smtp error while sending the messages from a particular name. And also I cannot receive mails to that email id;

    It shows an error like SMTP error (550) Failed to add recipient “” (verification failed for No such person at this address. Sender verify failed).

  • What is the solution for the error Rejected : Can’t submit your message, finished your day quota in way2sms.I need to send sms but msg doesnot delivered.what can i do?????/

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This article originally written on January 25, 2010, and is as relevant now as when it was first written.