Here’s how to detect Active Directory accounts that had an Exchange 2010 SP1 mailbox at one time, but the mailbox has since been disconnected, aka disabled.
Essentially find all accounts where a msExchWhenMailboxCreated value exists and a homeMDB value does not exist.
Get-ADUser -LDAPFilter "(&(msExchWhenMailboxCreated=*)(!homeMDB=*))" -Properties CanonicalName, msExchWhenMailboxCreated | sort CanonicalName | ft CanonicalName, msExchWhenMailboxCreated -AutoSize
– or-
Get-ADUser -Filter {(msExchWhenMailboxCreated -like "*") -and -not (homeMDB -like "*")} -Properties CanonicalName, msExchWhenMailboxCreated | sort CanonicalName | ft CanonicalName, msExchWhenMailboxCreated -AutoSize
This allows us to know before creating a mailbox for a user whether we should first bother to look for a disconnected mailbox that may already exist for the user.