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.
The workaround for Exchange 2016 is to configure the following registry keys within Windows Server 2016 and then reboot the server:
EnableHTTP2Tls = 0 (REG_DWORD)
EnableHttp2Cleartext = 0 (REG_DWORD)
Microsoft is actively working on addressing this within Exchange Online. In addition to the workaround above, if users are using a third party mail app on iOS (e.g. Outlook), they should be unaffected.
If you require any assistance, please reach out to us at firstname.lastname@example.org.
Update: Apple is reporting that they have fixed this in iOS 11.0.1. Their official KB can be found at https://support.apple.com/en-us/HT208136. Microsoft has not fully validated but I am hearing that initial testing is positive.