NOTE: This issue is resolved by installing iOS 11.0.1.
If you use the native "Mail" app on your iPhone or iPad to connect to your WWU email, you might want to wait before you upgrade to iOS 11. Apple and Microsoft have both acknowledged an issue that prevents the native mail app from sending or replying to emails for accounts hosted by Microsoft. You might see an error that says, "Cannot Send Mail. The message was rejected by the server."
We actually recommend the Outlook app over the native iOS mail app because it has proven to be more reliable. It also supports mailbox and calendar features that are not available through the native Mail and Calendar apps.
This issue impacts Office 365, Outlook.com, and Microsoft Exchange. Your WWU email is hosted by Office 365. If you have already upgraded to iOS 11, or you don't want to postpone upgrading until this issue is resolved, simply install the Outlook app.