Sysadmins: user leaves company but mailbox stays active with no OOO. What’s your standard approach?

Yknow what, my bad - you’re already gone. It should be easy enough for you to remember your contacts and send them an email. Or, barring that, reach out to them on LinkedIn for a connection with a short note that you’ve departed and would like to stay in touch. And there’s this thing called the phone that still works pretty well 😁 No matter what you do, keep your comms strictly to the “I’m doing something new and want to stay connected”. Doubly so if your employee agreement contains a non-solicit clause. You can get around this by asking for referrals to companies that might need help. If your contact wants to talk about work, only do it over the phone, too.