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.