Below is the powershell command to get the list of mailbox who last log time is older then 30 days. This would be very help ful when you wanted to try to clean up exchagne server from unused account. You can change from 30 to 6o or 90 days based on the requirement.

Code:
Get-MailboxStatistics | where {$_.Lastlogontime -lt  (get-date).AddDays(-30)} | Select  displayName,LastLoggedOnUserAccount,LastLogonTime