1. Open the EAC
Click on start and then expand Exchange Server 2016 and click on Exchange Admin Center.
2. Login to the EAC
If you have a certificate warning, click yes to continue and then enter the credentials to login and press enter.
3. Click on Recipients
On the left hand side, click on Recipients.
4. Select a Mailbox
Select a mailbox you want to modify and double click it or click on the pencil
5. Select Mailbox Features
On the properties page, click on mailbox features and then scroll down to where you see MAPI and click the disable button and then save.