Hybrid Server-Side Sync - Receiving 'InvalidSystemUsersOnEmail' for one user.

Hello,

I have one user in my system, newly setup, that is unable to send emails from CRM.  I have the org setup with Server-Side Sync for Incoming/Outgoing/ACT and when doing the Test/Enable, everything passed with "Success" messages.  Our setup for Server-Side Sync is hybrid with Exchange on-prem and CRM 2016 Update 1 online.   Has anyone else run into this error?  Can you let me know what it means and what I might be able to do to resolve the issue?

Note:  I've already tried disabling the mailbox, doing the approve email, and then testing/enabling the mailbox again.  Everything passed when I did this again but once again, no Outgoing emails will work through this mailbox.

Thanks!