You might not be able to send email with an Outlook.com, Office 365, or Exchange account until you update to iOS 11.0.1.
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: R20;Cannot Send Mail. The message was rejected by the server.”
To fix the issue, update to iOS 11.0.1 or later.