NOTE: Almost all major ISPs block port 25 (default SMTP port) these days, so they don’t relay smtp traffic).

To overcome this problem there are a few solutions:

1. Find out from your ISP's their SMTP server and use that.
2. Most of the ISPs still allow using port 465, which is the default secure SMTP port. If you'd like to use this you will need to do the following:
   Click 'Settings' in Outlook Express or 'More Settings' in Outlook. On the Advanced tab check the box under SMTP that says "This server requires an encrypted connection (SSL)" and change the Outgoing server port number from 25 to 465. The next time you send a message you will probably get a popup saying the certificate in use by the server couldn't be verified. I just click Ok.