Best Practice - Retaining Mailboxes Exchange Online

You asked for best practice about managing (preserving) mailboxes when users leave. The practice is different for on-premises Exchange than it is for Exchange Online (a fact to remember when reading up on the topic).

In 2015, Microsoft introduced the concept of inactive mailboxes to handle the problem of how to keep the mailboxes of leavers for what might be extended periods without requiring tenants to license those mailboxes. To make a mailbox inactive, you apply a hold to it (any hold will do), and then remove the Office 365 account. Exchange detects that a hold exists on the mailbox and moves it into inactive status. If necessary, you can recover data from or restore the inactive mailbox while the hold endures. As soon as the hold elapses,  Exchange marks the inactive mailbox as a candidate to be removed and the permanent deletion will happen soon afterwards.

The simplest kind of hold is a legal (or litigation hold). However, it might be easier to manage if you create an eDiscovery case with an associated hold. You can then add leavers to the hold to have it apply and make their mailbox inactive when their account is removed.

You can see the list of inactive mailboxes at any time by running the PowerShell command:

Get-Mailbox -InactiveMailboxOnly

This won't tell you the hold that is keeping the mailbox inactive - that requires more work.