I haven't messed with the defaults on undeliverable messages, but my recollection is that Postfix sends you a "hey, it didn't go through, but I'm going to keep

I haven't messed with the defaults on undeliverable messages, but my recollection is that Postfix sends you a "hey, it didn't go through, but I'm going to keep trying" email at 4 hours, and you get the final "I gave up" email about a week later.