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   iOS 11 Mail Failing to Send with Outlook.com, Office 365, and Exchange
   Accounts

   Mark H. Anbinder

   Some early adopters of iOS 11 are finding they can't send email via
   Outlook.com, Office 365, or Exchange Server accounts running on Windows
   Server 2016. According to a [1]support article from Apple, users with
   such accounts might see the error message: 'Cannot Send Mail. The
   message was rejected by the server.'

   Exchange Server and Office 365 accounts are common in corporate
   environments and at educational institutions, and Outlook.com offers
   free Microsoft-hosted mail service to anyone, much the way Google
   provide Gmail for free to individuals.

   In his [2]Essential Exchange blog, Michael B. Smith says, 'On a
   technical level, this happens because iOS 11 is improperly negotiating
   a HTTP/2 TLS connection and the connection fails.' He says iOS 10 and
   earlier versions of Windows Server don't have this problem because they
   defaulted to HTTP/1.1, rather than HTTP/2.

   On that Apple support article, dated 19 September 2017, the company
   said, 'Apple is working closely with Microsoft to resolve the issue and
   will release a fix soon in an upcoming software update.'

   [3]Microsoft's matching support article suggests downloading its free
   [4]Outlook for iOS app as an alternative. (Note that you won't be able
   to click that link on a Mac and download it; Apple's recent changes to
   the iOS App Store and iTunes 12.7 force you download it on your iOS
   device itself ' see '[5]iTunes 12.7 Giveth, but Mostly It Taketh Apps
   and Ringtones Away,' 15 September 2017.)

   Microsoft adds that if you're using an Exchange Server 2016 mailbox,
   not one of their Outlook.com or Office 365 mailboxes, 'you can ask the
   system administrator to [6]disable HTTP/2 in Windows Server 2016 as a
   workaround.'

   If you're not in a position to change your Windows Server configuration
   or ask your system administrator to do so, and you haven't yet upgraded
   to iOS 11, it might be easiest to hold off until Apple and Microsoft
   have resolved this issue.

References

   1. https://support.apple.com/en-us/HT208136
   2. http://www.essential.exchange/2017/09/18/ios-11-about-to-release-things-to-be-aware-of/
   3. https://support.microsoft.com/en-us/help/4043473/you-can-t-send-or-reply-from-outlook-com-office-365-or-exchange-2016-i
   4. https://itunes.apple.com/app/id951937596
   5. http://tidbits.com/article/17463
   6. https://support.microsoft.com/en-us/help/4032720/how-to-deploy-custom-cipher-suite-ordering-in-windows-server-2016

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