Greetings,
There are a few things we can test:
1) Are you sure the test address is bouncing? Try sending to it from Outlook. You should receive a bounce message a few moments later.
2) When testing mail to a list containing bounced address, are you sending enough email to exceed the current threshold set on the ListMail Config page? For example, if your handling is set to remove users if they bounce "3 times in 30 days", you'll need to send 3 emails to the bad address from ListMail.
3) On both welcome and standard mailings, check the "Source" of a message sent to your list. Near the top of the source, in the headers of the message, you'll see a line "Return-Path: <bounce@site.com>". Do your messages have the correct Return-Path?
Note: It is not uncommon for welcome messages to have the incorrect bounce address (can be fixed with a special config.php mod) while SMTP has the correct one. Welcome, confirmation, and notification emails all use internal PHP mail() to prevent too many connections to the SMTP server.
Regards
