iOS 11 Update - What You Need to Know BEFORE Updating

Apple has an advisory on their website (see https://support.apple.com/en-us/HT208136) that using email in iOS 11 may not work, and that they’re working on it.

We recommend you postpone updating until this is resolved if you use email on your iOS device.

UPDATE: Apple released iOS 11.0.1 to fix the issue-- you're good to update!