jdeane,
The following query will return all Exchange enabled users (i.e. with mailboxes) exluding system mailboxes and disabled users:
(&(!cn=SystemMailbox{*})(&(&(!cn=SystemMailbox{*})(!userAccountControl:1.2.840.113556.1.4.803:=2))(&(&(&(& (mailnickname=*) (| (&(objectCategory=person)(objectClass=user)(|(homeMDB=*)(msExchHomeServerName=*))) )))(objectCategory=user)))))
Regards,
Michael