Mail trouble with outgoing mail server

For weeks now (relatively recent), MAC mail will not send consistantly. When I choose my outgoing mail server in the preferences panel and check "use only this server", Mail often will not send my message. If I leave that outgoing server as "none", my first outgoing message of the day will prompt me to choose one, and then subsequent messages are sent without asking. Inconvenient at the least - something's not right.

Any help?