Message not sent because the server will not recognize your email

I have windows 7 and windows live mail.  I receive email but when I try to send a message, I receive an error message that states, " Message not sent because the server will not recognize your email."  I have called the server ( Cox-in N VA) and they went through their checks and referred me to Microsoft.  I realize that this is probably a minor problem for most folks but I am at wits end.  Thanks for any information.

[Original title: windows live mail]