1. Open Mail app
2. Click Mail in top left corner, then Add Account
3. Select Other Mail Account..
4. Enter following Details
Name: *name*
Email Address: *email address*
Password: *password*
5. Click Continue, then enter the following details
Email Address: *email address*
Username: *username* / *email address*
Password: *password*
Account Type: IMAP
Incoming Mail Server: mail.*DOMAIN*
Outgoing Mail Server: mail.*DOMAIN*
6. Click Sign In, then wait.
It may pop up with Can't verify identity, click continue
This may also ask for your password