Go to your iPhone Settings>Mail, Contacts, Calendars>Add Account>Other>Add Mail Account.
Type in your name, your email address, your password and description ( if needed ).
* Ensure IMAP is selected at the top left
Incoming Mail Server:
Host Name: mail.yourdomain.ext
User name: (your email address)
Password: (your password)
Outgoing Mail Server:
Host name: mail.yourdomain.ext
User name (your email address - must be done - not optional)
Password: (your password - must be done - not optional)
On the SSL pop-up, tap Yes
After about a minute, if you see the same SSL pop-up again. Tap Yes again.
You will then see the final Save screen, press Save in the top right corner.