Thoughtful, detailed coverage of the Mac, iPhone, and iPad, plus the TidBITS Content Network for Apple consultants.

iOS 11 Mail Failing to Send with Microsoft-Hosted Accounts

Some early adopters of iOS 11 are finding they can’t send email via, Office 365, or Exchange Server accounts running on Windows Server 2016. According to a 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 offers free Microsoft-hosted mail service to anyone, much the way Google provides Gmail for free to individuals.

In his 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.”

Microsoft’s matching support article suggests downloading its free Outlook for iOS app as an alternative. (Note that you won’t be able to click that link on a Mac and download Outlook; Apple’s recent changes to the iOS App Store and iTunes 12.7 force you to download it on your iOS device itself — see “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, but not an or Office 365 mailbox, “you can ask the system administrator to 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.


Try productivity tools from Smile that will make your job easier!
PDFpen: PDF toolkit for busy pros on Mac, iPhone, and iPad.
TextExpander: Your shortcut to accurate writing on Mac, Windows,
and iOS. Free trials and friendly support. <>

Comments about iOS 11 Mail Failing to Send with Microsoft-Hosted Accounts
(Comments are closed.)

Michel Hedley  2017-09-22 23:50
How could this happen? Was there no testing of iOS 11 with one of the most used emailers? Apple's software development keeps getting worse and worse. Every release of iOS seems to have issues that anyone could have foreseen. Maybe Apple should outsource some of its software development.
Scott   2017-09-26 12:02
funny I have iOS 11 and it is working fine with my Office365 account and has always worked even on the Betas? So how can some people have this issue and others don't?
I have experienced the same success with my Office365 account, so this is *certainly* not universal.
Mark H. Anbinder  An apple icon for a TidBITS Staffer 2017-09-26 15:38
Apple's support note for this issue has been updated to suggest installing iOS 11.0.1 to resolve it:

It's not clear why some iOS 11 users with, Office 365, or Exchange Server 2016 accounts were seeing the problem and others weren't, but we can hope this means no one will now!
I am still having this issue on iOS 11.0.2, but was not having it on 11.0.1... anyone else in a similar position? Can't get any clarity on how to fix it even after hours of Apple support and working with my office tech specialists.
Mark H. Anbinder  An apple icon for a TidBITS Staffer 2017-10-04 13:41
Yikes. For what it's worth, I never saw the problem with my work Office 365 account, and I'm still successfully sending mail on 11.0.2.