With the launch of iOS 11, a problem has been reported on some email accounts that are no longer able to send messages from the Mail app. Apple is working to solve the problem.
This error had already been noticed by beta testers during these months, but has not yet been solved with the final version of iOS 11. Apple has confirmed that the bug concerns both Outolook.com and Office365 email accounts, as well as generically hosted emails from the Exchange server.
The problem concerns sending an email: When you try to do this, the Mail app fails to send the message and reports the “Can not Send” error.
Apple is already working closely with Microsoft to solve the problem. At this time, the only solution is to access your email account through a web browser or to use an alternate email client.