This is really easy:
In PowerShell run:
Get-StaleMailBoxReport
If you want this in an excel file just pipe it out like this:
Get-StaleMailBoxReport | Export-CSV -Path C:staleMBs.csv
We can get a more detailed report with the following cmdlet:
Get-StaleMailboxDetailReport
Get-StaleMailboxDetailReport | Export-CSV -Path C:DetailedstaleMBs.csv