We set up a rule in our marketing department that if new leads come from LInkedIn, Google Adwords or other promotional programs, if they don't have a valid email address. I delete in Marketo. If it's a lead that does have a complete address, but is missing an email address but has phone number, we try to get the email and verify address. If a direct mail item is sent to them and it comes back to us, we delete the lead from SFDC and Marketo.
If it's a contact, and the person's email address has become invalid, our list developer tries to get a replacement email. If it's not available, remove the email address in SFDC.