Apple has acknowledged an issue with their native email app and these mail services. If you have already upgraded and are having a problem with email, you can download the Outlook app for iOS, which is not impacted.
Apple and Microsoft are working together to fix this issue as quickly as possible.
“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.”
Here is the link to their support article, if you want to keep tabs on this issue.
If you use the native email app, wait until you are prompted for an iOS 11 update that resolves this issue before you upgrade.