We have done batch submits of our email addresses to StrikeIron to validate the Invalid Email status, which helps keeps these to a minimum, but I hope to implement a real-time validity check with StrikeIron when someone submits a form.
Of course people still leave their company, etc., so we periodically if we have lead records that have the Email Invalid field set to true and that lead has had no activity in the last 6 months, we change the revenue stage from "Inactive" to "Bogus Info".
If we get repeated hard bounces due to spam blocks (e.g. Email Suspended for 24 hours), I check to see if the IP address for our shared Marketo email server is blacklisted (it rarely is). If it's a customer, we have our customer success rep contact them by Outlook email (sometimes these go through when Marketo sent emails don't) or by phone and we ask them to white list our email address / domain and email server IP, so that they will get our emails.