Ask HN: How do you archive old email accounts?

Like many of you, I run email servers (on-prem Exchange in my case). The same scenario always happens: a person leaves the company, three months later I get a frantic call, can I find the email this person sent to so and so?

What is your preferred method for archiving old email accounts before you delete them? Do you have a good method to search through archived accounts if you should ever need to?

For the past ten years I have felt strongly that there must surely be a better way than hanging on to brittle PST files or using Exchange/O365’s overly-complicated in-place archiving.