Whn you get the 'could not store' message everytime you're tfying to send a message, it looks like you've got no access to your Outbox.
I can think of three reasons for that.
1) It's read only (Did you per chance restore your message base from CD?)
2) You've got no writing access in the Outbox directory. This would be caused by creating an account as admin and later trying as restricted user.
(For both 1 and 2 it would be likely that you can't store newly received messages either.)
3) Somehow another program is blocking access to your Outbox. A likely candidate would be a virus scanner with you having somehow a virus stored as attachment in a message in your outbox. The remedy in this case would be to close TB, disable your virus scanner, delete the messages.tbb and messages.tbi files in your outbox directory, enable your virus scanner and start TB.
4) Your outbox might be corruoted for some reason. remedy is the same as in pt3, but without the virus scanner part.