There are some versions of IOS that have a "Trust this certificate" at the upper right of the screen.
Have them click it twice before the pop up message goes away.
For other versions of IOS that doesn't have the "Trust this certificate" follow the steps below:
- Check if they are using the host name as mail.theirdomainname.com
- If so,have them re-configured and use these settings:View mail client settings here