I'd make sure the bedding is such that rain/snow won't affect it and ask the person fetching your mail to feed it. If it has a home, it'll still be going home....you won't be stealing it. My guess is that if it's been sleeping at your door...and not just already visiting for food...that it's sending you a message that it's homeless. (It may have started out as someone's cat and ran away to escape something not right for it there...another cat attacking it, for instance.)
Thank you for taking care of it!