Step 1 - Go to Settings > Apps

Open Settings on your iPhone, scroll down and tap Apps.

Step 2 - Tap Mail

Step 3 - Tap Mail Accounts

Step 4 - Tap Add Account

Step 5 - Press choose from a list

Step 6 - Tap Add Other Account

Step 7 - Choose Mail Account

Step 8 - Enter your account details

Name: Your Name
Email: Your email
Password: Your email password
Description: a name for the account on your device
Then press Next

Step 9 - Enter your server settings

Enter the following settings, ensuring IMAP is selected as account type. 

Incoming Mail Server

•  Host Name: mail.yourdomain.com
•  Username: your email address
•  Password: Your email password

Outgoing Mail Server

•  Host Name: mail.yourdomain.com
•  Username: Your email address
•  Password: Your email password