Whoops: iOS 11 Mail Microsoft Issues

If you've upgraded your iPhones to the just-released iOS version 11 and your email is hosted by Microsoft's Outlook.com, Office 365 or Exchange 2016, you may now find yourself unable to send email from your iPhone. It sounds like a known issue and that it is being worked on.

In the mean time, the most commonly suggested workaround seems to be that you should download Microsoft's Outlook email client for iOS and use that instead.