Well, here's my first post on the forum :cool: ... I had the same problem with emails suddenly not working. I installed the email archiving tool mentioned previously ... then I went and looked at the archive and found "suspicious" information in the text of the email that was archived! (Also look in configuration>my store>store name to see if there's anything suspicious ... that's another big hint.)
The following thread gives indications of the "suspicious" looking stuff in the archived emails:
http://www.zen-cart.com/forum/showthread.php?t=133294
Turned out to be directly related to the Important Security Patch and vulnerability reported here:
http://www.zen-cart.com/forum/showthread.php?t=130161
So, I downloaded and installed the security patch ... cleaned up the "weird" php files in the images folder (read threads noted above) ... and reset the store name in admin ...
Then, for good measure, followed all of the security steps suggested here:
http://www.zen-cart.com/wiki/index.php/Important_Site_Security_Recommendations
:clap: THANKS to all the posters on the forum ... after many, many hours of trying to figure things out on my own, the "clues" from the email archiving and info found on the forum led to getting things working again. (Knock wood!)
I'm posting this HERE in hopes that it will help some people who, like myself, recognized an EMAIL problem but weren't aware of the SECURITY issues and clean-up required.