List hygiene
Keeping the mailing list free of dead, mistyped and disengaged addresses.
List hygiene is the routine of removing what should not be there: hard-bounced addresses, obvious typos, role accounts nobody reads, and subscribers who have ignored everything for a year.
It feels like throwing away audience, and it is the opposite. Receiving servers judge you by how people react; a list padded with addresses that never open drags down the deliverability of the ones that do.
The dangerous case is a spam trap — an address kept specifically to catch senders who mail people who never asked, or who never clean up. Hitting one damages your reputation quickly.