You need a powershell command which will get a list of all Microsoft Exchange user, where the latest login time is older then 270 days:

Get-MailboxStatistics -Server exchangesrv01 | where {$_.Lastlogontime -lt (get-date).AddDays(-270)} | Select displayName,LastLoggedOnUserAccount,LastLogonTime

Adjust the servername and the AddDays if needed, so that it fits your requirements