9/26: iOS 11.0.1 has been released and fixes the issue with ActiveSync with Exchange 2016\Exchange Online. See Apple article HT208136: https://support.apple.com/en-us/HT208136

HTTP/2 will now work on your Exchange 2016, running on Windows Server 2016, and with EXO.

1st heard of here http://www.essential.exchange/2017/09/26/ios-11-0-1-released-eas-on-ios-fixed/, by Michael B. Smith.

Apple released iOS 11 on 9/19/2017 and AGAIN they failed to test the largest email system in the world, Office 365\Exchange Online, with their email client. Apple has had a history of issues with Exchange since iOS 2.0 and with multiple iOS version have broken feature in Apple Mail on initial release.

So, if using an iOS device, do not upgrade to iOS11 yet, if your mailbox is hosted on O365\Exchange Online, Outlook.com, or if your organization is running Exchange 2016 on Windows 2016. The common factor here is that that Office 365\Exchange Online and Outlook.com all use Exchange 2016 running on Windows 2016.

The issue is that the native Apple Mail client in iOS 11 does not support HTTPS/2 TLS protocol, which is used by Exchange 2016. It seems, that Apple Mail can receive messages, but fails to send\reply to them. The Apple Mail App uses Exchange ActiveSync and when it connects to Exchange 2016, Exchange uses HTTPS/2 TLS by default, but Mail App doesn’t negotiate down to HTTP/1.1 and the connection fails.

In Office 365, you should see this alert MC119954:

If your organization is running Exchange 2016 on Windows 2016, you can disable HTTP/2 on the server. To do this see this Microsoft article: How to deploy custom cipher suite ordering in Windows Server 2016, which just has this RegKey setting:

To enable and disable HTTP/2, follow these steps:

1. Start regedit (Registry Editor).
2. Move to this subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters
3. Set DWORD type value EnableHttp2Tls to one the following:
   1. Set to 0 to disable HTTP/2
   2. Set to 1 to enable HTTP/2
4. Restart the computer.

If your mailbox is hosted on Outlook.com or Exchange On-line and you have already updated to iOS11 your only option, currently, is to change email clients. I HIGHLY recommend Microsoft’s Outlook App (download here).

Other articles on this issue:

• Microsoft: You can’t send or reply from Outlook.com, Office 365, or Exchange 2016 in iOS 11 Mail.app (Article ID: 4043473)
• Apple: If you can‘t send an email with iOS 11 and an Outlook.com or Exchange mail account (HT208136)
• Tony Redmond’s: iOS 11 and Exchange 2016/Online: Not Kissing Cousins
• Michael B. Smith’s: iOS 11 about to release – Things to be aware of