[conspire] missing rDNS for (intentionally missing) IPv6

[Rick Moen]

Quoting Michael Paoli (Michael.Paoli at cal.berkeley.edu):

> Hmmm, did it reject with a "permanent" failure?

SMTP 550 result codes, so, yeah, permanent rejects.  Not 45x, which
encourages retries. 

I was blissfully unaware of that until I tried to send a direct mail to
Ross, CC'ed to Leo.  (They are both on GMail.)  That was my wake-up call
about the problem's existence, right around 1:30pm, today.