Split from this thread.

Whenever Windows Live Mail fails to send a message, it will display an error message explaining what's wrong. Please remove any unsent messages from the outbox (below the account folders in the folder pane), then compose a new test message with a unique subject and send it to yourself. Post the error message in your reply. You can click on an error message to select it, then use right-click to copy it and paste it into the text entry box.

Hi, I am having the same issue, although I didn't have it when I first set things up. It has only started recently. This is my error message...

An unknown error has occurred.

Server: 'smtp.gmail.com'
Windows Live Mail Error ID: 0x8007007A
Protocol: SMTP
Port: 465
Secure(SSL): Yes

Hope you can help :-)