The ability for Nimble to automatically check my contact's email/s through a ping process. This service exist today but it requires you to do them one at a time or by exporting your list and importing it and then manually going to each contact to delete the email addresses that are bad.

Placing a warning indicator next to the email address and having somewhere on the dashboard that shows the number of bad email address would be great. You could click that and bring up the list of the contacts that have a bad address.