Newer Versions of iPhone

In the directions below, wherever it says "yourdomain.com" substitute with your domain name.

1. Tap "Settings" app
2. Tap "Mail, Contacts, Calendars"
3. Tap "Add Account..."
4. Tap "Other" (bottom option)
5. Tap "Add Mail Account"
6. Enter Name, address (full email user@yourdomain.com), and password.
   (iPhone will attempt to guess your settings. Allow ~1 minute for it to fail.)
7. Select IMAP.
   (It's the default. The option you've selected will be highlighted blue.)
8. Under "Incoming mail server"
   1. Set "hostname" to mail.yourdomain.com 
      (or your server host name or server's IP address)
   2. Set "username" to email@yourdomain.com
   3. Set "password" to your password
9. Under "Outgoing mail server"
   1. Set "Host name" to mail.yourdomain.com 
      (or your server host name or server's IP address)
   2. Set "User name" to email@yourdomain.com
   3. Set password to your password

You are now ready to send and receive emails via your iPhone.