Getting Email Delivered
  - The Email Deliverability Blog

Most legitimate mail senders know to process bounce messages. This means that if they get what is called a “hard bounce” for an email address on their list (meaning that the bounce tells them that the email address simply doesn’t exist), they remove the email address from their list permanently (and immediately, right guys?)

If they get a soft bounce (such as “mailbox full”), they may sideline that email address and try it again in a week or so.

Failure to remove bounced email addresses - particularly hard-bounced addresses - will result in your email being blocked. That’s because repeatedly sending to non-existant email addresses is taken by most ISPs as a sure sign that either a) you are sending to very old lists, and you don’t care, because you’re a spammer, or b) you are sending to a list that you ‘acquired’ from somewhere else, so you’re a spammer, or c) you’re a spammer.

So clearly processing email addresses that bounce and getting them off your list as quickly as possible is a Very Important Thing to Do.

Again, most legitimate email senders know to do this, and so as soon as they get that email bounced back to them saying that the address does not exist, they remove the email address.

But..guess what.

ISPs have started not bouncing back an email when there is an attempt to deliver to a non-existant email address.

Instead, they are inserting the bounce message right into your mail log, right at the time of the SMTP transaction (in other words, when your server attempts to transmit the email to their server).

Which means that if you try to send an email to xyz@example.com, and xyz@example.com doesn’t exist, you will never see an email bounce. Instead, you will see something like this in your mail log:

550 <xyz@example.com>, Recipient unknown

Now, the upside to the ISPs doing this is that if a spammer sends out a spam run, and uses your return address in the “From:” field (making it look like the spam came from you), your server will not be crippled by all the bounced email messages - as can and does happen sometimes.

But, it also means that you need to know to start looking at your mail logs, every day, for these rejections, and to treat them just like any other bounce. Which means get that email address off your mailing list, asap.

Otherwise you’ll look like a ..well, you know..and your mail will get…you know.