When you forward or reply to an email when using the native iOS 11 mail application with Exchange ActiveSync against MailEnable running on Windows Server 2016 you receive an error message "Cannot Send Mail. The message was rejected by the server."
This is cause by a bug in the iOS 11 release, which was resolved in 11.0.1.
This was resolved in iOS 11.0.1. Upgrading iOS will resolve the problem. It can also be resolved by disabling HTTP/2 on the Windows server. This is done by adding the following Windows registry keys and rebooting the server:
|Product:||MailEnable (All Versions)|
|Class:||PRB: Product Problem or Issue|
|Revised:||Friday, September 29, 2017|