I can receive mail on my iPhone, but I cannot send!! Help!!

Ok, so you've got your iPhone and you've got it setup to receive email fine but, despite your best efforts, you simply cannot get it to send. Well, you're in luck and you're not alone. This article assumes that you're using a POP3 or IMAP account on your iPhone and that everything else is setup as it should be (e.g., you're able to receive email on your iPhone).
To send mail via SMTP using your iPhone on the AT&T 3G network, do the following:
- Go to Settings
- Click Mail, Contacts, Calendars
- Choose your email account
- Click on SMTP
- Under Other SMTP Servers, click on AT&T SMTP Server
- Turn it on
- Click the Home button
- Enjoy sending emails!!

- Some additional background on this article is also available here
- Some Apple / iPhone specific information is available here