There is a system smart list in Marketo for 'Bounced email addresses' which basically uses email invalid filter.
Do you just delete these email addresses from database or leave them in there as marketing suspended?
For us, we also add activity of any kind as a filter and look at timeline, and whether they've been deleted from SFDC or not. Sometimes if it's a more recent bounce, there may be data associated with that record from web visits and email activity within the last year that we want to keep and have a look at for Account/Opportunity mapping purposes. It's really depends a lot on your business and reporting.
It depends on where they are in the funnel (does sales have a working number for the person? Are they part of an opp?) or if they were previously an engaged lead (if it was a good lead, but then they moved to a new company, you may want to find their new info and merge with the existing lead).
If it's just junk, like a bad email from a form or list purchase, or really old and outdated and hasn't popped back up somewhere else, I would delete.