A few years ago we migrated to O365 which means now I have a bunch of old employee mailboxes wasting space on my Exchange server.
I want to reclaim that space by expunging the mailboxes but I don't want to delete them outright. Is exporting to PST the proper way to do this?
An alternative method I thought of, would be to script the creation of a database for each mailbox, move the box, and then wait a day for a good backup. I could then archive, dismount, and delete all the databases.
As above, if you have E3 (or Exchange Online Plan 2) licences available to cover temporarily licensing those users, you could consider migrating them, licensing them as E3 and placing them under in-place hold or legislative hold, then deleting the user.
This is known as the Inactive Mailboxes feature, and described here:
It is the standard approach for organizations that need to keep employee mailboxes after they leave, and it is not uncommon to migrate on-premises mailboxes that fall into this category and follow this procedure.
Apart from the license pool required to support migrating those mailboxes in batches before removing the licences and re-using them, you don't need to continue to pay for those E3 licences and if it's a one time procedure can then re-use those licenses as you desire.
If you need to restore data or access data from Inactive mailboxes the typical approach will be to use Office 365's compliance center to search and export that data.
Keeping a mailbox database is a more risky strategy, as you may find it hard to re-mount this in the future, and instead need to rely on a third-party tool to export the mailboxes, particularly if you remove all Exchange Servers in the future and the users that the mailboxes relate to.
Exporting to PST is OK; but you risk losing items in the Dumpster (Recoverable Items) when you export, and if the data is required to be restored for legal reasons, lose chain of custody (i.e. the paper trail to prove messages were not removed or changed without oversight). Plus, you still need sto