If you have just updated to iOS 11 and are using the native iOS mail app to connect to your Microsoft Exchange 2016 Server running under Windows Server 2016, then you’ll have to deal with several problems like not being able to send / receive mails.
The cause is that iOS 11 now uses HTTP/2 to communicate with Exchange. HTTP/2 is also the preferred method of Server 2016 to communicate with clients when Exchange ActiveSync is being used. It seems that there are problems with the correct implementation of HTTP/2.
The current workaround is to disable HTTP/2 on the Windows Server.
This can be done in the registry under
by creating the following keys:
EnableHTTP2TIs = 0
EnableHTTP2Cleartext = 0
After rebooting the server HTTP/1.1 is being used for communication. Rumors say apple is working on an update.
Since Tuesday 09-28-2017 this error has been corrected with iOS 11.0.1