I have these settings: imap.sonic.net port 143

outgoing server: mail.sonic.net port 465

Thanks again, Nyla

When you changed the password, did you remove the old one from TB, then restart TB and enter the new one when prompted?

Not sure it matters, but I'd recommend using secure settings for both incoming and outgoing:

imap.sonic.net on port 993, SSL/TLS, normal password, User Name = part of address before @

mail.sonic.net on port 465, SSL/TLS, normal password, User Name = part of address before @

https://help.sonic.com/hc/en-us/articles/115000384388-Thunderbird

I checked with sonic and tech support indicated that although I used different ports on computer and phone this was not a problem. The issue seems to be Thunderbird. . . I used the identical email address for reply to my member via gmail and the email was sent with no problem.

Thanks

The rules for sending through webmail are less strict than sending on smtp. I think the length of the user name (76 characters) exceeding 64 is probably why it doesn't work with TB.

https://help.returnpath.com/hc/en-us/articles/220560587-What-are-the-rules-for-email-address-syntax-