And, is there a minimum number of times that an email bounced before a lead enters this list? Trying to determine best practice re: deleting these leads from my Marketo database vs. keeping them in (could they come off the bounced email address list?)
The System Smart list is by default filtered to Email Invalid = True.
This should be hard bounced when an email is undeliverable. But there are different reasons for bounces and what it causes, this might help to give more answers:
Other than the docs, it's a good idea to decide on the process for your firm. There are several threads here, and on my site, on what to consider and how to build out automation to keep the list clean.