This guide explains how to add your email account with IMAP to the email app on your iPhone.
Step 1 - Go to Settings> Mail
Open Settings or Réglages on your iPhone and type Mail.

Step 2 - Tap on Accounts
Tap on Accounts.

Step 3 - Tap on Add Account
Tap on Add account.

Step 4 - Tap on Other
Select Other as the account type.

Step 5 - Tap on Add Mail Account
Tap on Add Mail Account.

Step 6 - Enter your account details
Enter the account details of the email you want to add:
Name: this will be visible as the sender.
Email: the address you want to add.
Password: you use it to connect to webmail.
Description: A name for the account on your device.
Tap Next to continue.

Step 7 - Select IMAP
Tap on IMAP as the account type.

Step 8 - Enter your server settings
Enter the following parameters:
Incoming server
Host name: mail.example.com
Username: your email account
Password: the one you use for webmail
Outgoing server
Host name: mail.example.com
Username: your email account
Password: the one you use for webmail
Tap Next to continue.

Step 9 - Register the account
Disable Notes sync, if enabled. Tap on Save.

Your account is now correctly configured.