Normally in the mailqueu of the mailserver of your hostingprovider.
If you have root rights on the servers, you could have a look.

The next version of this mod will do a safety check to prevent unalterable users from being managed for bounces.

So when I have 500 emails bouncing back to me I would have to click this link in every message or?

Yes, you would

Yes but if you set it up properly you'll only have to do this for one complete run through your database. The first few days, I was clicking through batches of 50, I think (the number is selectable in your ACP). After that, I get perhaps one or two a week.

What about members that are in more than one usergroup? Such as with some paid subscriptions?

And users that have forgotten their password. Will the password reminder still work for them?

And what if they fix the bouncing problem. Will they be able to keep the same email in their profile, only reactivate it?

Members in more than 1 usergrup shouldn't be a problem as this will only change their primary usergroup.

Pasword reminder should still work as well.

As far as keeping original email, that shouldn't be a problem either since the bouncing members email is not changed, only the option to receive emails.

this clashes with the inactive users hack.

Inactive User Reminder Emails
Found 3 Inactive Users.

Sent To: **********

Fatal error: Call to a member function on a non-object in /home/******/public_html/*******/includes/class_mail.php(261) : eval()'d code on line 4

Actually, it doesn't. Uninstall 1.10 or 1.09 of the inactive users add-on. Download and install version 1.09. They work just fine together.

Chris is working on the issues with versions 1.10 and 1.11. Use 1.09 in the meantime.

So we would need to enable PM for users in the Waiting for Email Confirmation group?