To prevent spam, many ISPs block external outgoing mail servers configured for port 25. You can check to see if your ISP blocks external SMTP servers by running a simple test:

1. Click "Start" in the lower left corner of your computer screen.
2. Select "Run" and enter the following:

   telnet smtp.bizmail.yahoo.com 25

If the port isn't being blocked, you'll receive a response starting with "220". If you do not receive a 220 response, your ISP is blocking outgoing mail server port 25, and you will need to change the SMTP port from 25 to 465 using SSL (recommended) or 587 without SSL.

To learn how to update the SMTP port, consult the email program's help files or the POP Settings instructions for your program.