I'm currently testing a CRM 2016 Environment and have restored using a back-up of our CRM 2013 database, so all our email settings are same.
When I Test & enable mailbox, both the Outgoing Email & Appointments, Contacts and Tasks work successfully.
The incoming test doesn't seem to work and it creates the 
error. When I check alerts, there is nothing showing.
Has anyone encountered this before?
We are using CRM2016 On-Premise and using server-side sync.