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.
That seems like your problem.
Outlook Express has the ability to provide the SMTP server with your name & password as well as the POP3 / IMAP server - does Tbird really not have something similar?
Have you checked tbird support forum?
Is there another multi-account email package that I could use instead?
What do you mean / require by multi-account? OE allows you to add more than one email account per identity (with discrete inbound/ outbound server settings) and also allows you to have multiple identities, each with the ability to set up their own inbound/ outbound server settings - you can then switch between them when required (File\Switch Identity).
I'm not advocating OE (although it is fine for me!) - my father swears by (not at) Pegasus Mail, and others like Eudora... and of course there are other mail packages.
Why do you want to go to tbird if Outlook XP works for you?
SD