It is recommended that you enable the failure notices for your mailing lists before sending messages. This will keep you informed when one of your mailings are rejected or fail.
Here's how to enable this feature:
1) login to the ServerTools control panel
2) Click on Email Manager -> Mailing Lists
3) On the Mailing Lists page, click on the wrench icon next to your mailing list:
4) At the bottom of the mailing list page, click on the following checkbox:
5) Click on the Update button at the bottom of the page.