Setting up your new email account on your iphone:

1. Click 'Setting'.
2. Click 'Mail, Contacts, Calendars'.
3. Click 'Add Acccount...'.
4. Click 'Other'.
5. Click 'Add Mail Account'.
6. Fill in the fields & click the 'Next' button.
7. Select the 'IMAP' button and fill in the rest of the fields with the information below:
   
   Under the 'Incoming Mail Server' section enter the following:
   'Host Name'
   Enter mail.yoursite.com for your Host Name.
   'User Name'
   Enter your e-mail address again.
   'Password'
   Enter the password you set up for your e-mail account. 

   Under the 'Outgoing Mail Server' enter the following:
   'Host Name'
   Enter mail.yoursite.com for your Host Name.
   'User Name'
   Enter your e-mail address again.
   'Password'
   Enter the password you set up for your e-mail account.
   Now click the 'Save' button

8. Under 'Accounts' click on the new account you just created.
9. Click 'Account Info'.
10. Click 'Advanced'
11. Click on 'Deleted Mailbox'.
12. Select 'Trash' under 'On My Iphone', then click the 'Advanced' button.
13. Click the 'Advanced' button at the top of the screen.
14. Next to 'IMAP Path Prefix', enter 'INBOX' & click the 'Account Info' button.
15. Change the 'Server Port' to 143.
16. Click the 'Account Info' button at the top of the screen.
17. Click 'Done' and exit the app.

Now whenever you click on your email icon, you can see your new account in the list of mailboxes. If you can't see the list it's because you're already in an email account. Click the top left button, with the name of your email account in it, to return to the 'Mailboxes' screen.

Apple Support Page