I have been using Thunderbird for a number of years.
As of yesterday I have been unable to send outgoing messages via Thunderbird.
This happened immediately after I changed my BT Internet password - yet I am still receiving incoming emails.
It may be that you need to update your SMTP password in Thunderbird as well. In Thunderbird, the credentials for the incoming (POP3/IMAP) server are stored separately from those for the outgoing (SMTP) server, so if you changed the password for incoming mail, you may have to update it for outgoing mail as well.
Thank you for your advice - seems the problem was as per your email response. Having to change passwords for both incoming and outgoing mail seems incredibly long-winded and unnecessary?? ... Fortunately a friend/colleague of mine was able to give me a hand and my system is now back up and running again. It does however make me wary about any future password changes ...