This procedure is applicable for iOS devices (iPhone, iPad, etc.) running iOS 11 and higher. The screenshots shown below were taken from an iPhone 7 running iOS 11.0.1. Other devices will be similar.
If you have iOS 11 and experience issues with email connectivity please update to the latest version of iOS 11. Apple has acknowledged an issue with the Mail app and email connectivity affecting Exchange in the following article: https://support.apple.com/en-us/HT208136
Prior to starting this procedure, authenticate your account and obtain your account-specific information from the Customer Portal according to the following article:
How to Obtain Exchange Account Information
To setup an Exchange email account on an iPhone or iPad take the following steps:
1. From the Home screen tap Settings.
2. Scroll down and tap Accounts & Passwords (Passwords & Accounts in iOS 12).
3. Tap Add Account.
4. Tap Exchange.
5. Type in the Email address and account description (optional) then tap Next. Note: The description labels the account on the device and is not visible to anyone else.
6. Tap Configure Manually on the prompt that appears:
7. Enter the settings shown below. If these don't work, the most up-to-date settings can be obtained from the Customer Portal.
8. Choose the Exchange services to synchronize. If prompted, tap Keep on My iPhone.
Note that other types of iOS devices will display similar confirmation notices.
If data is stored in the device's local address book or calendar these options are turned OFF by default. Synchronization of these services can be enabled, but a pop-up window will appear prompting whether to keep or delete the local data. Unless you are sure you don't need this information, the Keep on My iPhone option should be chosen.
9. Tap Save.
10. The account is now added! Email should appear in the Mail app within a few minutes. Please allow some time for all synchronized data to download.
If you ever need to update any of your exchange account settings for this device, please refer to the following article:
How to Update Exchange Account Settings on an iOS 11 Device