I cannot send from one specific BT Mail account but can access folders using IMAP.

Question

Thunderbird 31.3.0 on WIndows 8.1 64 bit.
I have two email accounts configured into Thunderbird.
Both are BT accounts and have identical configurations.
IMAP is used for reading mail, SMTP for sending mail.
On one I can read folders, delete emails from the Bulk Mail folder, fetch and send mail.
On the other I seem to be able to do everything except send mail.
I can read mail, delete SPAM, but for some reason not send emails.
Both accounts are using "mail.btinternet.com" as the IMAP and SMTP server.
I am using Thunderbird as a back up email client to deal with a very few messages which cause major problems with my main email client which is Windows Live Mail 2012. I have double checked and the mail server details are the same between the two and my WLM client can send emails to the same SMTP server on the same port as Thunderbird fails on.
I know the password is correct because I can connect over IMAP and delete SPAM using Tbird.
The only difference that I can see between the two accounts is that one is "@btinternet.com" and the other is "@talk21.com".
The symptoms - I see the progress bar with "Connected to mail.btinternet.com..." and green all the way across.
Some time later I get a separate warning saying "Login to server mail.btinternet.com failed".
Because of the time delay I am assuming that the session has timed out.
I am also assuming that if the credentials were wrong the rejection would have been immediate (although the delay may be to reduce the effectiveness of brute force attempts to guess the password).
So I know that the IMAP credentials are O.K. - I can access the mailbox.
I know that the SMTP credentials are O.K. - I can send mail from WLM using the same settings.
Further confusion - I know that it worked in the past because I have a test message in my Sent mail from when I first set up Tbird.
This is all very weird.
Has anyone else seen something similar?
Also, is there a debug option where I can trace the SMTP protocol when the email is being sent?
Cheers
LGC

Answer 1

Update: - I sent two separate test messages and saved both the failing emails as Draft to retry after restarting Tbird.
I've just noticed that they are both in a Drafts folder on my mail server.
Not, however, the Drafts area for emails composed using the web mail client but under a separate Folders tree.
I can't send the email with the web mail client, although I can forward it.
So further proof that the IMAP side is working.