You can't send or reply from Outlook.com, Office 365, or Exchange 2016 in iOS 11 Mail.app
You can't send or reply from Outlook.com, Office 365, or Exchange 2016 in iOS 11 Mail app
Apple says it is working closely with Microsoft to fix an issue that prevents Outlook.com, Office 365, and Exchange 2016 accounts 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.
Microsoft published a similar support document urging users to instead use the Outlook App for IOS