Prerequisites

In order to install an SSL certificate on iPhone/iPad, you need:

Process

Open your Webmail in your Safari browser.
Tap on the Integration with device link.
Select the SSL certificate option. Enter your username and password if prompted.
Tap Allow to download the profile.
Tap Close once the profile is downloaded.
Open Settings and tap on Profile Downloaded. Tap Install.
Enter your passcode and tap Install twice.
Tap Done.