It's difficult to notice that something isn't there.
It used to be that, if an email couldn't get to its intended recipient, then it would be "returned to sender". We called this a bounce, and it was useful, because at least you knew that something had gone wrong. Maybe a mis-spelled email address.
But I've just noticed that these days, I get very few bounces. Of course, it may be that I've become really good at not mis-spelling email addresses, but I don't think that's the reason.
It's Joe Job.
Because of the practice of Joe Jobbing, anyone who was the victim of a Joe Job, would get hundreds, thousands or even hundreds of thousands of bounces. Not useful, not at all.
So I think what's happened, is that many people have reconfigured their email system so as to not bounce undeliverable emails - it just fails, and you don't know about it.
The way I handle this, is that I accept all email, but then my despam filter has a list of valid email addresses, and if the email was sent to an address not on that list, then it goes into a "not-for-me" area, which I occasionally look at and delete. So I don't bounce messages.
One dide effect of this, is that when something is sent to a huge bcc (blind carbon copy, and I remember when carbon copies really were carbon) then one of my valid email addresses isn't there, and it gets put into "not-for-me". But I'm happy with that, because I doubt if I really want to read someting that has been sent to a large number of people.
I'm guessing that lots of ISPs have done something similar, so that if you send an email to a user at that server, and the user doesn't exist, then the email is silently ignored.
Which is good.