By default Apple's iPhone enables TLS (encryption) for incoming and outgoing emails. A valid TLS certificate is only available on a dedicated server/VPS.
1. Enter Mail on the iPhone
2. Select Other from the list of mail provider options
3. Enter all the Account specifics:
Host Name: mail.yourdomainname.com
4. Click Save, and get the invalid certificate message
5. Click "CANCEL", an you get returned to the settings screen
6. Click "SAVE" again, it says, "You may not be able to receive email..."
7. Click OK
8. Now you can go back into the settings, the ADVANCED button now shows up at the bottom of the mail screen.
9. Now you can go into the advanced tab and turn OFF SSL for both sending and receiving mail.