I only want to pull Mailbox Enabled Users from my Active Directory Source

When you have selected to sync Source User objects, by default, all objects with the mail attribute populated will be pulled (Mail Enabled Users). This may include Mail Enabled Users that do not have a mailbox assigned.

To pull only Mailbox Enabled users, add the following filter to the Source tabs Optional LDAP Query Filter for User(s) Query:
(msExchHomeServerName=*)

Save the changes on the Source tab, and then run Discovery. You will see that only Mailbox Enabled users are discovered on the Source.