Do not send emails when hard bounced

[joshsmith]

Problem

If a user's email has hard bounced, we need to ensure that we do not attempt a send to that email.

[begedin]

Note that after a switch to Sparkpost, this should not be a problem. The sparkpost API tracks bounces and blocks sending of problematic emails automatically. It then provides a way to get a list of those emails, which we could use to filter out or flag problematic users.