I can receive emails just fine, but I can't send emails. Why not?

In an attempt to control the spam problem, some Internet service providers block port 25.

If your ISP is blocking port 25, you'll be able to receive emails, but not send any.
To get around this problem, please change your email program's settings so that it uses port 587 instead of port 25.

If you're not sure how to do that, check with the help files in your email program or simply go to http://google.com and search for a phrase like "how to change port 25 to 587" for more information.