Your ISP may be having trouble. My best advice is to call them and NOT CHANGE A THING ON YOUR MACHINE.
In fact, many look for help at the first sign of such an error and don't call their ISP, wait an hour or worse, begin to fix their machine.
It might not be your problem.
When I try to send messages I get an error message that has a yellow triangle with a black exclamation point in the center of it. The message reads" Some errors occured while processing the requested tasks. Please review the list of errors below for more details. The only thing I can find is in the bottom of the message under a tab "Tasks" is Status is WARNING. Have XP can receive mail and surf the internet.
CANNOT SEND MESSAGES.