E-mail messages not delivered occasionally when sent through Thunderbird

Hi. I have a very odd problem with sending e-mail messages from my home computer using Mozilla Thunderbird.

The messages appear to have been sent normally, but then a few days later I receive an e-mail (not always) in my spam folder from mailer-daemon@comcast.net with the subject line "Permanent Error".  This e-mail has a *.eml attachment with my original message and says:

"This is an automatically generated Delivery Status Notification.     

Delivery to the following recipients failed permanently:"

This problem does not seem to happen with every message I send.  However, I have no way to tell if the messages are received since it looks like they were normally sent.  Other times, I don't even get the mailer-daemon message in my spam folder.

 I have no issues when I use the web interface of Gmail or the phone app.

Any ideas?

Thank you