This weekend I upgraded all the macs in my house with the handy dandy 5 pack of Snow Leopard.
Both my wife and son use morelweb.com domains with Mail.
After the upgrade no email could be sent.
Cox locks down sending email over standard ports via any mail server but the cox mail server.
All emails that we tried to send were rejected with the error message:
The sender address "xxx@xxx.com" was rejected by the server

I took a long time to figure out the very easy fix for this.
Edit the SMTP server list.
Find the entryfor smtp.west.cox.net
Click custom port
Enter 25 for the custom port
Save everything
Send email

hope this helps someone.