Getting Outlook Express to Send Email Over VPN

Recently I have subscribed to a VPN connection.  While connected to the VPN, I tried to send email using my Outlook Express, I recevied an error message:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'xxxxxxx@gmail.com'. Subject 'test11.11', Account: 'mail.xxxxxxx.com', Server: 'smtp.xxxxxxx.com', Protocol: SMTP, Server Response: '550 5.7.1 <xxxxxxx@gmail.com>... Relaying denied. Proper authentication required.', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

It just refused to send out my email.  But once I disconnect myself from the VPN and connect just with my normal ISP internet connection, I was able to send the email without problem.

After doing some research, I came to a solution.  All I need to do is to set my out going Mail [SMTP] port to "587'" instead of the usual "25".

And yes the mail sent out without any problem.