[Spinball]

tamarian, yes, that sounds great provided the error codes can be 100% identified for emails which have bounced. A '550 Invalid recipient' would require the user to be moved to requiring email activation (with ideally a PM sent to the person but email notification of that PM not) whereas a '552 5.2.2 Over quota' would need to be processed as per your quota system.
The big problem I have is when we send out an email to all our users (40,000 ish recipients). We get something like 1,000 or 2,000 bounced emails, with a lot of those being from AOL (it must detect a lot of emails from a single source and block them all), and maybe 300 or 400 which require the user's account to be deactivated.
This plugin could potentially save me 2 or 3 hours every time we send out the mailshot.

[Edit - just had a thought. What about people who mistakenly reply to thread notifications, as they sometimes do? They think they are replying to the thread. It's usually less experienced users doing it.
Ideally they need an email back saying that they have mistakenly replied to the forum mailer.