Mail Program Malfunction

Hi,

My Mail (version 2.1.3) suddenly started rejecting my sender email address. It will receive mail but not send it. I went to account information and retyped my email address, retyped my outgoing server, restarted the Mail program, and rebooted, but it's still doing it. The error message says, "The sender address [my email address] was rejected by the server."

There is no problem if I use web-based mail. What else can I try?

Thank you!
Annette

More info: It's a POP account; I tried Connection Doctor and it was fine...um, what else can I tell you?

Thank you!

Annette