Recently when I try to send mail to friends with a "." in their ID I get this message.  There's no problem when I use my ISP webmail and there's no problem with other IDs.

An unknown error was returned from the SMTP server.

Subject 'test'
Server Error: 579
Server Response: 579 message content is not acceptable here
Server: 'mail.wavecable.com'
Windows Live Mail Error ID: 0x800CCC61
Protocol: SMTP
Port: 25
Secure(SSL): No