Thanks and sorry forgot to mention that we are on CRM 2016 on-premise with Update 1. All users have access to the record to Account/Contact/Case etc. that they are supposed to be able to set regarding to the email.
I've checked that other activity type like Task, PhoneCall and Appointment, we can set regarding to Account/Contact/Case etc. Email is the only one that we cannot set regarding to anything else other than Account for some reason.
I did look everywhere in our javascript on the email forms that might have prevent this, but there is no such thing.
Some users are able to set regarding to Case from Outlook email, and some users don't. Most users who are unable to, are new users.
Any input is greatly appreciated...
-tri