A better heuristic for undeliverable subscribers

We've tweaked how we handle consecutive soft bounces to be a little smarter.

Previously, we would mark a subscriber as undeliverable if they had three consecutive soft bounces in the past seven days — the logic being that if a temporary failure (as defined more by the SMTP protocol and our upstream vendors) is happening for a week straight, it's not very temporary.

This logic has one flaw, which is that many people don't send three times in a given week. In particular, many people only send emails once a week. So we've tweaked the logic to also check for four distinct soft bounces in the past thirty days.