Message could not be sent, server response: 550 Sender is not allowed, error 0x800CCC78

Hi Support,

I got the same problem,

For nothing the mail can't send out seen yesterday.

It show me this

The message could not be sent because the server rejected the sender's email address. The sender's email address was

Subject '123456'
Server Error: 550
Server Response: 550 Sender is not allowed.
Server: 'mail.westech-security.com.sg'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 25
Secure(SSL): No

Split from an unrelated thread