From this thread: Archived discussion on email problems ...
It actually has nothing to do with Zen Cart or the unroutable domains I was sending mail to. It has to do with cPanel configuration.
In cPanel, ...
Sometimes your mail server configuration requires a particular format in order to send messages.
Options to try include:
Zen Cart doesn't handle email/newsletter bounces automatically.
If you get email "bounces" coming back to you, you can turn ...
In the /email folder, you have a file called "header.jpg".
Simply change the contents of this file in order to reflect the new logo in the HTML emails you ...