They should also get a reason why. There is no use guessing here. Are the messages in your Inbox if you log into your account at the providers Webmail site? If not ask your provider where these messages are.

This is the mssge senders get back: Delivery to the following recipients failed permanently:

 * comcastmailss@gmail.com

Reason: Permanent Error This is not a real email address btw.

Tell the sender to fix the address for you. They send it to the wrong address. Your providers server rejects it. There is nothing your or Thunderbird can do about it.

I'm afraid that is not the answer. This address: comcastmailss@gmail.com is a totally bogus address. People write me at my actual email address: zymrgist@comcast.net, and they get the above response in return. It seems, somehow, attached to my actual address and has only cropped up in the last couple of months.

Then contact your provider. The error message is from their server and not Thunderbird.