Disallowed breakage found in header name - potential virus

About every 6 months or so, I have a problem with outgoing messages being stopped by my ISP’s Email Scanner. “Problem found in email message you sent. Email scanner intercepted and stopped message”.
When I look into my Eudora Options, I see where my Mail Server has been changed to 127.0.0.1, and my LogIn name has had a pound key symbol (#) PLUS my mail server name inserted AFTER my login name. As a result, my ISP’s scanner gives me the “Disallowed breakage” note as stated in subject, and stops my email as “potential virus”.
I understand Avast needs to change the server , but WHY the login name.?
When I change my settings back, everything fine.
Any ideas.? I’d hate to remove the program.
Thanks in advance . . . . frank :o :o

This is indeed a confirmed bug in the latest update - see this thread for example.
A fix should be released very soon.

However, you are saying that you’re having the problem for months? That’s really strange…