There is a script which will query your Active Directory infrastructure for all Exchange servers registered, and create a detailed CSV report of all mailboxes in the organization. Have a check.Noya Lau TechNet Community Support

HI, How to identify how many mailbox user in exchange 2003? Also how many group email in Exchange 2003? Thank you. jaie

Go to ADUC, under Action Toolbar menu choose Find. On the Exchange tab select users with mailboxes.Thanks, Swapnil Prajapati

I would suggest you to create a new Search Query and select users with mailboxes/distribution groups. The result can be exported as csv for further processing in this methodRegards from ExchangeOnline | Windows Administrator's forums