Just recently I've been having issues with email on my primary a/c on Thunderbird.
(I know I know but it still doesn't explain it) I am running XP SP3 with Thunderbird 9.0 (the latest for my system) and have several email addresses setup, some from other providers and others being secondary btinternet(bt/yahoo platform) addresses.
All work initially then the primary a/c fails with the above for instance when writing an email and it won't save the draft, or other times it refuses to send even though it will if I select the SMTP of another server.
I've deleted the a/c from Thunderbird and then recreated it allowing the auto-detected settings to take place with the above result again.
The recalcitrant a/c is working fine on my Android phone and on the web interface.