I recently converted from using the CRM 2013 email routing to using server side synchronization. We have an on premise exchange server and it's only one organization.
All the Test & Enable Mailboxes work for everyone. People receive the test email and it is properly syncing the calendar etc.
Our processes that send email and when you try to send email directly through CRM get stuck "Pending Send" though. For example, if I'm in a case, go to activities, and try to send an email this happens. I can't seem to find anything under any Alerts etc. so I'm not sure where to look to determine what is causing this.
I did the Application Impersonation in Exchange and like I mentioned the Test & Enable works fine.