dns resolution failure for EMAIL_HOST blackholes messages

[jsnshrmn]

I ran into an issue where the DNS entry for the specified host stopped resolving to anything. In this case, all messages marked for sending ended up with a status of draft. I'm assuming that's because djmail knew better than try to send them in the first place, which is fine. The problem is that there was no way for me to resend once the underlying issue was resolved without manually updating the status to something that the resend management command could use (I chose pending).