I recently went through and disabled a ton of users that hadn’t logged into AD for the last X number of days. And now I needed a report to show the number of mailboxes that were currently associated with disabled accounts.

Exchange Powershell to the rescue
Get-Mailbox -ResultSize Unlimited | ft -a Identity, UserAccountControl, ExchangeUserAccountControl, isLinked, isResource > c:\temp\disabled_mailboxen.txt