You need to delete the password Thunderbird has remembered. http://kb.mozillazine.org/Setting_and_changing_email_passwords

Thanks, but that didn't work. It's rather silly that you can manage the server password from the outgoing server settings panel. But anyway, I did find it (found Passwords under "Security", not "Privacy", though), deleted it, still doesn't work, no password prompt, just fail.

I tried to provide feedback on that link article as well, since it is no longer correct, but that's apparently a completely separate login? Anyway, that should be corrected, since it's also misleading.

Sorry it did not work for you the first time you tried but that is how it is done.

Tools-Options-Security-Passwords-Saved Passwords

Remove ALL the saved password for that account. They might be there more than once. Once for inbound and another for outbound.

Remove them both. Close and restart Thunderbird. It should ask you for a password the next time one is required.

I have just spent several hours wrestling with the same problem.

Why the Thunderbird path Tools>Options>Security>Passwords>Saved Passwords does not casually mention that to change server provider passwords, simply:

'Remove ALL the saved password for that account. They might be there more than once. Once for inbound and another for outbound. Remove them both. Close and restart Thunderbird. It should ask you for a password the next time one is required.'

confounds Mozilla's usually excellent signposting, IMHO

Thanks for the inspiration, Airmail