Discard notification emails if the recipient’s email address is unconfirmed

Discard notification emails if the recipient’s email address is unconfirmed

This change discards notification emails if the recipient’s email address is
unconfirmed after the email was created and before the email was actually seat.

An example might be that the background queue is delayed, due to the mail
server’s spam protection kicking in, allowing the admin to unconfirm email
addresses of email addresses that are no longer valid to prevent more bounces
from being generated.