Apple, Microsoft Working to Fix iOS 11 Mail App Issues With Outlook.com, Office 365 & Exchange Accounts
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.
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Top Rated Comments
Same happened in the past, it's not like it's unusual.
As for the thing about the app store in iTunes, obviously Apple would have real data about that. It's probably true that very few people use the app store on the desktop, otherwise they wouldn't have removed it. Personally I removed it from the iTunes menu because it was illogical to download mobile phone apps on a desktop when I don't even sync my phone with the desktop. Sure, there will no doubt be a small number of people who were still doing this, just like there were a small number of people using Dashboard widgets or DVD drives. That's just the way it goes, unfortunately, same for Google when they kill services and features, same for Microsoft when they abandoned their phones.
I get that they don't use it.
That said I have been managing Exchange servers since the Exchange version 4....24 years ago. When the iPhone first came out it did not have Active Sync support, they wanted Exchange admins to turn on IMAP. It might have happened in a few places but for the most part the iPhone did not work in the corporate world.
Then Apple licensed Active Sync and that was one of the BIG reasons Enterprise adopted the iPhone after that. To not test this thoroughly test Active Sync against Microsoft's email offerings it a mistake on Apple's part. Microsoft still owns the Enterprise Email market and will for a long time. Apple has dumped on the Enterprise market enough over the last 5-10 years. Doing stuff like this only gives those MS fanboys in most Enterpise IT shops more fuel to the "Apple does not do Enterprise" fire. They are probably trying to push some Android option.
This is NOT the first iOS update to break Active Sync support.
[doublepost=1505911674][/doublepost] Its not every O365 tenant. just the ones that Microsoft turned on HTTP/2. They are rolling the affected servers back now. plus, it was only with emails with attachments or replies with a signature with an icon.