outgoing mail server rejects mail

Hello,

My ISP's SMTP server started rejecting the outgoing mail I am trying to send through it. I called them, but they checked the settings with me (which as far as I know had not changed). They said they were OK and that I should call Apple.

Does anyone know what would cause this I restarted the computer and the modem/network completely. It still does it. I have a pc on the same network which is able to send mail through the same SMPT server, so it does seem like something on this mac maybe. Any suggestions?

Thanks

Bob