Apple says it is working closely with Microsoft to fix an issue that prevents Outlook.com, Office 365 & Exchange 2016 Accounts from sending or replying to emails using the Apple Mail App in IOS11 after the initial launch of IOS 11 on Tuesday September 19th 2017.
The Message will be “Cannot Send Mail. The message was rejected by the Server”.
To workaround the issue you will need to download the Free Outlook App from the App Store and use this instead of the normal Mail Client.
If you have any further issues please email firstname.lastname@example.org or call our support team on 01246 861100