Handling of bounced email addresses - and removal of them from mailing lists - is perhaps one of the least understood and yet most crucial of practices. The failure to remove bounced email addresses from a mailing list in a timely manner can lead to erroded delivery of email to the point of being completely blocked by some ISPs.
Senders should mark an address as "dead", meaning the sender should remove the address from the delivery list and not attempt to deliver to the address again unless and until the sender has reason to believe that delivery rejection would not occur, if a given email address incurs a hard bounce (such as "user not found" or "no such user"), or if the address incurs three consecutive "soft bounces" (such as "mailbox full" or "try again later") and the time between the most recent consecutive soft delivery rejection and the initial consecutive delivery rejection is greater than fifteen days.
The bottom line here is don't keep trying to send email to email addresses that don't work! It's poor practice, and it understandably pisses off the ISPs.
It is also not sufficient to rely on email bounces - that is bounce messages that come back to you and end up in the sender's inbox. Many large ISPs now do not generate bounce emails, and instead the bounce messages will be in the sender's email logs. Therefore it is critical to regularly scan your mail logs for such bounce messages.