How to Search Emails Across Multiple Mailboxes in Office 365?

For checking emails still in mailboxes, you’ll need to use eDiscovery through the compliance center. You can do this by running `Connect-IPPSSession` from the ExchangeOnlineManagement module. If you're more interested in the flow of messages rather than what's in the mailboxes, you should go for Message Trace. However, keep in mind that for instant reports you can only go back 10 days, and downloadable reports can look back 90 days, which might take a couple of hours to generate.

You can definitely use eDiscovery in Purview for searching emails. However, if you're looking for something simpler, utilizing Message Trace might be the way to go. It typically provides the information you need without the extra complexity.

Don't forget that if you’re needing to trace messages, message trace can help you out. Just a heads up, though: there have been some recent changes regarding how long back you can go. You can set up a loop with a script to look back further, but it’s still limited to about 10 days at a time.