The new iOS 11 Apple update was released this week and some users are experiencing issues with Office 365 and Exchange 2016. If you use the built in iOS email client and updated to iOS 11, you might see an error message that says “Cannot Send Mail. The message was rejected by the server.” Apple has released a statement that they are working closely with Microsoft for a resolution, and that the issue will be corrected on the next software update. For additional information, see Apple’s announcement.
Alternatively, the free Outlook for iOS app in the Apple App Store has no issues with the update and can be used as a workaround until the fix is applied.