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 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 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 Outlook.com 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.