Mobile devices that are running iOS7 and later versions cannot automatically configure the default mail app for an Exchange Online mailbox or on-premises Exchange Server mailbox. When a user tries to verify their account, a message is displayed that says that their identity can't be verified by Exchange.
The Autodiscover request for the secure root domain (https://contoso.com/Autodiscover) has an untrusted certificate. The root domain presents a certificate that doesn't match the HTTPS root domain name.
Make sure that the certificate matches the user's SMTP domain, or manually configure account settings for the Exchange Online mailbox.
Article ID: 3150466 - Last Review: Aug 10, 2016 - Revision: 1
Exchange Server 2016 Enterprise Edition, Exchange Server 2016 Standard Edition, Microsoft Exchange Server 2010 Enterprise, Microsoft Exchange Server 2010 Standard, Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard Edition, Microsoft Exchange Online