If your company uses Office 365 for business email, don’t upgrade your iPhones or iPads to the newly released iOS 11 just yet. There is a known issue in iOS 11 that will prevent the Mail app from sending or replying to Office 365 emails.
Apple and Microsoft are working on a fix, so hold off on that update button until one is released. If you use the Outlook app on the phone instead of the Mail app, you are not affected by this bug. If you have already upgraded and normally use the Mail app, you can install the Outlook app from the App Store to send and receive mail until the bug is resolved.
You can read the advisories from Microsoft and Apple at the following link:
Microsoft Support – You can’t send or reply from Outlook.com, Office 365, or Exchange 2016 in iOS 11 Mail.app
Apple Support – If you can‘t send an email with iOS 11 and an Outlook.com or Exchange mail account
If you are affected by this bug or have questions about upgrading, you can contact me at firstname.lastname@example.org or 954-908-3999.