Critical Issue with Apple iOS 11 Mail App, Exchange 2016/Online and Windows Server 2016
Apple has announced that iOS 11 is releasing on Tuesday, September 19th, 2017. If you are running Exchange Server 2016 on Windows Server 2016, or using Exchange Online, please keep reading. If not, this issue should not affect you.
The major symptom that users of the native mail app will experience is the inability to reply to or forward emails. This critical issue occurs when the native iOS 11 Mail app connects via ActiveSync to Exchange running on Windows Server 2016 (thus only Exchange 2016 or Exchange Online are affected). Prior versions of Windows Server are not affected. The cause of the issue is an incompatibility between iOS 11 and Windows Server 2016 relative to an attempt to leverage HTTP/2 and the associated conversion made within Windows between HTTP/2 and HTTP/1.1.