Notification if No Email Address Associated with Requester

When we receive tickets via Talk, the requester is a phone number. My agents get the name of the caller and update on the ticket. If they don't get the caller's email, Support still allows them to follow up via email. Even though there's not an email associated to that customer record. 

The agent assumes the caller received their follow-up when they didn't receive anything at all. 

It would be nice if Zendesk notified the agent when pressing Submit that there's not an email associated. Or worst case, sent a delivery failure notice to the support brand. At least they'd know the email wasn't sent.