iOS 11 - Can't send/reply from Outlook.com, O365, or Exchange 2016 in iOS Mail.app?

I have a workaround that seems to work:
Go to the Accounts & Passwords in Settings, select the exchange account, then click on the account and retype the password in the field. When done you should see the checkmarks indicating all the info checked out and then you should be able to send emails again.

Gabe Shackney
Princeton Public Schools

The worst I've seen is that sometimes emails go to the outbox but never get sent that was over the weekend on the GM. if I go to the outbox and send from there, then they go through. I haven't found anybody that has run into that today.