There are tens of millions of spam traps online. These are email addresses that when sent to flag you as a spammer because the only way you could have gotten it was through harvesting, and no double opt in permission is ever given.

There are a few companies that somehow (I have no idea) can detect these traps and bad addresses and scrub your list, taking them all out.

My guess is that there are inside people who have access to these trap databases and can constantly scrub for all of them.

I pay $400 per million emails I have scrubbed by one of these companies. I don't spam, but I buy websites with big lists attached, acquire companies lists (in other ways), and I always have them scrubbed.

Scrubbers also take out all the bad emails for you so you have an up to date list.