Currently iOS 11 has issues with ActiveSync accounts that are connecting to a Server 2016 (IIS 10) web server, services such as Outlook.com, Office 365 and possibly other systems as well. We had hoped Apple would have resolved this semi-critical issue before the public release, unfortunately though, nope.
For ASP1 SmarterMail customers using ActiveSync, we applied the "workaround" which involves disabling HTTP/2 on Server 2016 about 2 weeks before iOS 11 was publicly released, so customers who have upgraded already will not experience any email interactions with our services.
If you have not upgraded and you use email with Outlook.com, Office365, we recommend that you wait until Apple has released an update that resolves this.
This issue only affects ActiveSync and the Apple Mail app when connecting to Windows Server 2016, it does not apply to Outlook for iOS. We have also received reports stating that the K9-Mail app for iOS is also not affected. If you are using IMAP or POP, this also does not affect you.