Apple is reporting that after upgrading to iOS 11 some users are receiving the following error when they try to send emails: "Cannot Send Mail. The message was rejected by the server."
Details of this issue can be found on Support.Apple.com. Apple is working with Microsoft on a resolution.
If you have users who are impacted by this, they may want to leverage VMware Boxer as a workaround. Email should flow normally from Boxer.
To fix the issue, update to iOS 11.0.1 or later.