To resolve this issue, install your Enterprise Root CA Certificate on the iOS Device. You can do this manually or by using the Apple Configurator. The cause of each message is slightly different, but both errors are caused by the inability to verify the authenticity of a certificate or certification authority.

Certificates are moved to the device via email. You will need to export a password-protected .pfx file, and email it to yourself. Then tap on that .pfx file to install it onto the device/Outlook mobile. The following image shows how manual certificate delivery works in iOS.

Send digitally signed messages as clear text if you want to make sure that your digitally signed messages can be opened by all recipients, even if they do not have an S/MIME mail application and can't verify the certificate.

On iOS, certificates are stored in the publisher keychain. On Android, they are stored in the system keychain.

When you send a digitally-signed macro or document, you also send your certificate and public key. Certificates are issued by a certification authority, and like a driver’s license, can be revoked.

Open the digitally signed message. Look at the Signed By status line to check the email address of the person who signed the message. It isn't enough to check the email address in the From line—you want to verify who actually signed the message, not only who sent the message.

If you do not want to purchase a digital certificate from a third-party certificate authority (CA), or if you want to digitally sign your document immediately, you can create your own digital certificate.

Synchronization of contacts between your Outlook account and your Apple iPhone or iPod touch requires Apple iTunes. You can configure which items are synchronized during a one-time set up process.

Microsoft Intune delivers certificates automatically to Outlook for iOS. To obtain or renew your certificates, please follow the instructions in the notification or email you received from Company Portal.

This solution enables you to resolve the issue quickly for individual Mac clients without having to reissue the certificate. When you receive the certificate warning message, click Show Certificate.