I've just run into this problem and found a work around on the 'net. Mozilla 1 and Netscape 6 & 7 reportedly have this bug and obviously it still persists in Mozilla 5. just found it in bugzilla:
http://bugzilla.mozilla.org/show_bug.cgi?id=146488
Likewise I could ping and telnet (port 25) to the SMTP server but trying to send would result in an error: unable to contact SMTP server. Other accounts on the Linux box were able to send (with exactly the same SMTP configuration).
The solution (or rather work around): (untested) edit prefs.js file (see bugzilla report) and remove duplicate/invalid smtp server entries
or
keep adding duplicate SMTP servers. Edit:Mail & Newsgroup Account Settings:Outgoing Mail Server: Advanced make sure all have the "use name and password" box unchecked and select different copies as the default and eventually it resolves itself. I had to add 4 copies of the SMTP server and edit the "null" one which appeared on its own and voila I could send e-mail again. :-S
Hopefully you've solved this by now (given it's 2 months on) but maybe it will help someone else.