Everytime I try and send a message to a group/category I get the following message coming up:

An unknown error has occurred.

Subject 'Barry Hilton Live! '
Server: 'smtp.gmail.com'
Windows Live Mail Error ID: 0x800CCC0B
Protocol: SMTP
Port: 465
Secure(SSL): Yes

If I am replying or just sending an email to one or two people they get sent but this error only occurs when I try and send to a category.

Please help!