iPhone Certificate installation process for Office 365
After the migration to Office 365, user certificates required for encryption will no longer be able to be pulled from the directory. This means certificates will need to be saved locally and attached to a contact item in iOS in order to be used. To restore encryption capabilities, follow this process:
Find an email that is digitally signed by the user you are attempting to send to. This is indicated by the check mark or the padlock symbol after the name. Once the message is located, tap the name of the user in the From field..
This should display the user information and have a link to view the associated certificate.
Tap the View Certificate link and the certificate will display. It may show untrusted. Tap the Install button and allow the certificate. This will cause the iPhone/iPad to hold on to that certificate and have the ability to encrypt any time a message is sent to that user.