Apparently there is a problem with the Golden Master (GM) of Apple's iOS 11 with the Apple Mail App and Exchange Server 2016 on Windows Server 2016. In this case it is not possible to reply to mails or forward the mails with the Mail App.
iOS 11 is not yet publicly available, but it is due to be released on 19.9.2017. Whether there will be a fix by then is currently unknown. So before users start complaining that they can no longer receive or send emails from September 19, the workaround below can be used.
The cause of the problem is related to the implementation of HTTP/2 in Windows Server 2016 or iOS 11 (currently not entirely clear). Windows Server 2016 prefers HTTP/2 over the older HTTP/1.1 by default. With iOS 11, Apple now uses HTTP/2 in the Mail app, which leads to the problem described above. Older Windows Server or iOS versions are not affected by the problem.
As a workaround, the following two keys can be created in the registry:
- HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters
- EnableHTTP2Tls = 0
- EnableHttp2Cleartext = 0
The two keys switch off HTTP/2 for Windows Server 2016, HTTP/1.1 is then used again.
After creating the keys, the server must be restarted. To restore the default setting, the two keys can be deleted again.
Update 09/26/2017: The iOS update 11.0.1 fixes the error. Once all devices have been updated to the latest software version, the registry key can be deleted again.