If iPhone is unable to send mail, try these basic troubleshooting steps.

1. In Settings, turn on other SMTP servers.
2. Add your email account on iPhone instead of syncing your mail settings from your computer
3. Turn iPhone off and then on by pressing and holding the Sleep/Wake button for a few seconds until the red slider appears, then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears.
4. In Settings, turn off Wi-Fi to only use the cellular data network.

If iPhone is unable to receive mail, try these basic troubleshooting steps.

1. If you are using one or more computers to check the same email account, that may be creating a lock-out issue that is easily solved.
2. Add your email account on iPhone instead of syncing your mail settings from your computer.
3. Turn iPhone off and then on by pressing and holding the Sleep/Wake button for a few seconds until the red slider appears, then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears.
4. In Settings, turn off Wi-Fi to only use the cellular data network.

If the steps above don't resolve the issue:

Arrange a phone call with an Apple Expert who specializes in your exact question. They will even call you! Get started here. (Note: Callbacks are currently available only for U.S. and Canada phone numbers.)

Get a free GMail, Yahoo, or AOL account and forward your mail to that account.

See the iPhone User Guide.

Although this article is specific to iPhone, many of the steps also apply for iPod touch.

[Source: Apple Technical Articles]