Apple says it is working closely with Microsoft to fix an issue that prevents Outlook.com, Office 365, and Exchange 2016 account holders from sending or replying to emails using Apple's native Mail app in iOS 11.
Apple published a support document on Tuesday, September 19 – the official launch date of iOS 11 – to acknowledge the issue affecting users of Microsoft email servers.
If you can‘t send an email with iOS 11 and an Outlook.com or Exchange mail account
You might see an error message that says "Cannot Send Mail. The message was rejected by the server."
If your email account is hosted by Microsoft on Outlook.com or Office 365, or an Exchange Server 2016 running on Windows Server 2016, you might see this error message when you try to send an email with iOS 11: "Cannot Send Mail. The message was rejected by the server."
On Tuesday, Microsoft also published a support note, which states that Apple's Mail app in iOS 11 "is not compatible with Outlook.com, Office 365, or Exchange Server 2016 running on Windows Server 2016".
Both companies' statements suggest they were caught off guard by the Mail app problem in iOS 11, however MacRumors is aware of several users who installed beta builds of the mobile operating system and flagged the issue with Apple as far back as July.
Despite apparently being aware of the problems for some time, both Apple and Microsoft said they are still working together to resolve the issue. Apple added that it would release a fix soon in an upcoming software update, while Microsoft offered the following workaround in the meantime.
To work around this issue, download the free Outlook for iOS client from the App Store. The Outlook for iOS client fully supports various email services, including Outlook.com, Office 365, and Exchange Server 2016.
If you use the Mail app to sync data from Exchange Server 2016 that is running on Windows Server 2016, you can ask the system administrator to disable HTTP/2 in Windows Server 2016 as a workaround. The instructions on disabling HTTP/2 can be found in the Workaround section of KB 4032720: How to deploy custom cipher suite ordering in Windows Server 2016.
Stay tuned to MacRumors to find out as soon as Apple releases a fix for the ongoing Microsoft server issues in iOS 11.