List hygiene means maintaining recipient data so you are not repeatedly sending to addresses that should not receive mail.
At minimum, process permanent delivery failures, honor unsubscribes, investigate unusual complaint rates, and keep subscription sources understandable.
Permission comes first
A technically clean list can still contain unwanted mail. Do not buy or scrape addresses and then call the result healthy because the addresses happen to exist.
Avoid fake engagement
Do not try to manufacture opens, clicks or replies to manipulate reputation systems. Build a stream of mail that recipients actually requested and can easily stop.