If you support a large Exchange environment, it's likely that users come and go frequently. In this case, it's quite possible that over time, you will end up with multiple unused mailboxes. In this recipe, you will learn a couple of techniques used when searching for inactive mailboxes with the Exchange Management Shell.
Finding inactive mailboxes
How to do it...
The following command will retrieve a list of mailboxes that have not been logged on to in over 90 days:
Get-Mailbox -ResultSize Unlimited | ? { `
(Get-MailboxStatistics $_).LastLogonTime -and `
(Get-MailboxStatistics $_).LastLogonTime -le `
(Get-Date).AddDays(-90)
}