An interesting piece of advice that I saw in another HN thread about good company behaviours stuck with me over the years. When someone unsubscribes from your mailing list, delete their email address from your database. Don't retain it marked "inactive" or "unsubscribed" so that you can accidentally or on purpose start emailing them again later. Really delete it, so that you no longer have it and CAN'T make that error.

But then it can be accidentally re-added to the mailing list since there's no history of the opt-out.

If the person chooses to opt back into the mailing list (all mailing lists should be double opt-in) then of course they can be re-added. There is nothing wrong with quitting a mailing list and re-joining sometime later, if that is what you want to do.

But then we can't "accidentally" re-enable it later?!