Do you delete invalid email addresses?

Delinda has a good point above.  We can't delete a lead in Marketo that has other valid information because some of our programs are integrated.  So our sales have a chance to contact them and validate their email. 

It seems like lots of people have issues with this. I am also curious how to handle this. We are looking to do exactly what Delinda is trying to do. This is an older post, so I'm curious if anyone has come up with a solution. We don't want to delete the lead just make the email address NULL. We have recieved a ton of  "domain name does not exist" errors and wanted to be able to alert our salespeople so that they can clean them up or enter new ones.

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.