I know a lot of my customers have Yahoo Mail and I'm sure a lot of readers do as well.
If you don't have a business account ... you may have a few problems checking your email on your iPhone. This application
has a direct/web based interface
to Yahoo mail.
You can also access all Yahoo services through the new iPhone app:
Check it out here:
* Link above will open iTunes, or allow you download directly from the iPhone / iPod Touch