How to Identify and Clean Up Unused Exchange Online Mailboxes

Here's a start for you! You can use this command: `Get-Mailbox -RecipientTypeDetails SharedMailbox | Get-MailboxStatistics | Select DisplayName, LastLogonTime, TotalItemSize`. It should give you info about the last logon for those shared mailboxes, even though keep in mind that technically, those mailboxes don't have individual logins.

There's a last used property with `Get-MailboxStatistics`, but beware, it's not super reliable. You'll need to combine info from `Get-Mailbox` and `Get-MailboxStatistics` and maybe even do an audit search for mailboxes to get the last actions. Focus first on mailboxes without access permissions or those forwarding to users who left. It's a smart way to filter out active ones quickly.

For a thorough process of elimination, start with conducting a message trace on each mailbox for any messages sent or received in the last day, three days, and ten days. If you find no activity, then check the Unified Audit Log for any activity within the last 30, 60, and 90 days. Just be prepared, it can take a few days to run all those reports, but you’ll clearly see who’s using what!

You might not be using Exchange Online directly, but you could try the Graph API's cmdlet: `Get-MgReportMailboxUsageDetail`. It allows you to export to CSV and look at the LastActivityDate, which can help you identify inactive mailboxes. Check out the official documentation for more on this method! [Microsoft Docs](https://learn.microsoft.com/en-us/powershell/module/microsoft.graph.reports/get-mgreportmailboxusagedetail?view=graph-powershell-1.0)