Please forgive me if my comment/suggestion sounds overly simple, or patronizing, as I do not know for sure from your post whether the "user" that is "unknown" is you or the addressee, and you did not indicate whether you are sucessfully sending mail to others using Thunderbird.
Having said that, one difference between sending via Thunderbird (imap or pop) and either the webmail interface or gmail, is the potential for confusion when "authenticating".
Thunderbird pop or imap requires you identify yourself with your "m123455" user, not your "user name" as used in webmail or gmail.
If you are sucessfully sending email to any others using Thunderbird, then this obviously is not your problem, but if not, you might check that.
If you are only getting the errors when sending to one addressee, then I'm stumped!