I'd love to see an organic email verification service (pings email servers to check for existence of accounts) as an extended service for Campaign. I'd imagine the service work like this:

1. Verify any new addresses added to a given database (could be all databases, could be a specific database, such as a crm-integrated database)

2. Update a field in the database for the recipient with a status of the verification (valid, invalid, etc.)

3. Setup an automated routine to re-verify addresses after a certain amount of time has passed (12 months, 18 months, 24 months, etc.)

This service could be an added service with a set pricing structure that companies could opt in to (like Inbox Monitoring).