This is kind of a very big bug on iOS 11. Many user have complained that they cannot send , receive, and replay from their Outlook.com, Office 365, or Exchange 2016 in iOS 11 Mail app on iPhone iPad, after upgrading to iOS 11. Apple official has already announced this problem and that they will team up with Microsoft to solve this iOS 11 exchange issue.
By far, the practical solution is downloading 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. Hope it works for you.
For more iOS 11 problems, you can find the answers in this page, I found it couple days ago: 37 Common iOS 11 Problems and Quick Fixes >