https://arstechnica.com/gadgets/2017/09/apples-ios-11-0-1-update-addresses-exchange-e-mail-server-issue/

Apple has released a small software update for iPhones and iPads today. iOS 11.0.1 "includes bug fixes and improvements for your iPhone and iPad," but the short note provided to iOS users when they update doesn't specify which fixes or improvements they can expect.

However, Apple has claimed in its support documentation that one of the most jarring problems with last week's iOS 11 release is addressed by this update: the Exchange e-mail server bug that prevented sending e-mails for many users relying on e-mail accounts hosted on Outlook.com, Office 365, or certain Exchange Server 2016 configurations. Users received an error message saying, "Cannot Send Mail. The message was rejected by the server."