If your email account is hosted by Microsoft on Outlook.com or Office 365, or an Exchange Server 2016 running on Windows Server 2016, you might see this error message when you try to send an email with iOS 11: "Cannot Send Mail. The message was rejected by the server."
Apple is working closely with Microsoft to resolve the issue and will release a fix soon in an upcoming software update.
We'll let you know when a fix becomes available. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.