I get this error, I run a forum so there are 1000s of notification emails that go out everyday, I get this error often. What can be the problem?
are you using q-mail with your forum software...please give the details of the set-up you are running
Most likely the email address user's have signed up with aren't reachable any longer, and when confirmation messages are sent to their account, they fail to deliver (hence why you get the message)