I'm with Ms. Cuddles on this one. Sounds like an unfortunate set of coinky dinks...

I infrequently receive legitimate return email (from work and my yahoo account) that say the mail server from company xxxx cannot receive my email for various reasons. I also am sure that I did not send the email. Most likely someone is spoofing my address to try to appear legitimate.

Many email clients (sounds like your version of mozmail as well) will ask for confirmation if sending in text (or html) format if that is not your selected default. Usually you can look in prefs and figure out how to turn that "feature" off.

As always, my recommendation would be to use text as your default format. More devices and software packages can read that format, and there is less opportunity for someone to monkey around with potentially nefarious html code.