Zen Cart does not filter out any email addresses.

However, what Zen Cart "does" do differently than your regular email programs ... is send email directly from the webserver. It's possible that this webserver has been blacklisted by bellsouth.net due to spam sent from other domains on the same server.
This would be akin to your personal address being blocked for spamming, which would prevent you from successfully sending to those addresses from your personal PC.

It's possible that blacklisting happened during the time between switching versions.


I suppose if you're convinced that the problem is with the 1.3.5 version's email system, you could replace the functions_email.php file with the previous version and see whether it works any differently...