Some examples below is granting full mailbox permissions to users without the auto mapping feature.

Run the following command to grant full mailbox access to a single mailbox:

Add-MailboxPermission –Identity <mailbox> –User <user> -AccessRights FullAccess -InheritanceType all -Automapping $false

Or, to give a user access full access permissions to all mailboxes, run:

Get-Mailbox | Add-MailboxPermission –User <user> -AccessRights FullAccess -InheritanceType all -Automapping $false