Keef - many thanks for your help. I'd already tried the untick option, and no joy. I use TBird for five different email accounts that are on three different ISPs, including BT. My Outlook XP (not Express) collects mail from my business ISP, but sends quite happily out on BT, and looks like this:
One thing I have noticed is that TBird does not have an outgoing server password box. The send failure seems to occur without a password being requested by the BT SMTP server.
While I like TBird, I'm not wedded to it. Is there another multi-account email package that I could use instead?