Go to settings > Mail > Accounts > Add Account > Other > Add Mail Account
Enter a name: This is just the name of the mailbox
Enter your email: name@yourdomain.com
Enter Password (case sensitive): Your Password (case sensitive DO NOT COPY AND PASTE)
Hit Next
Enter incoming mail server: mail.yourdomain.com
Enter username: name@yourdomain.com
Password (Fill in if not already filled in) Your Password
For the outgoing mail server you do exactly what you did for the incoming mail server (Password will need to be typed in for outgoing)
Hit Next/Save
Your email account will show up on the mail app that comes with the IPhone.