To make a long story short, some users at another company who I have a lot of email correspondence with started having problems sending me email, they would get a message from the mailer daemon at MessageLab (which hosts their email) that
Delivery is delayed to these recipients or distribution lists:, etc. etc. Since the message was coming from their server and nobody else was having a problem sending me mail, I figured the problem must be on their end, but just to be sure I checked with Rackspace (who hosts our email). Sure enough, none of the affected emails had hit our server at any point, which isn't surprising at all because none of them ever actually left their server[s] at MessageLab as it turns out. According to what MessageLab is telling them, UltraDNS handles Rackspace's DNS and UltraDNS has blacklisted some of MessageLabs's IPs, with the end result that their server is unable to do a DNS lookup for our MX, which is why the mail is not getting sent, much less delivered.
My question is does this make any sense? My knowledge of the workings of DNS is high-level at best, but it seems unlikely that a) there would be a blacklist on doing DNS lookups,or b) that even if the authoritative nameserver did have such a blacklist that it would come into play all that much due to the distributed nature of DNS. I realize of course that there are blacklists of IPs used to filter spam and the like, but my understanding is that that's something that the server receiving email would use to do a lookup of the sender's IP, not something that would affect the originating server's ability to query the DNS system. But maybe I'm wrong?
Edit: In case it helps, here's an example of the message log that shows the failure; this is what was sent to me from the tech person at the other company -- I think it's the output from an admin panel, but I don't really have any more info than what's below:
2013-01-23 06:13:25 Connection from: xxx.xxx.x.xxx 2013-01-23 06:13:25 Sending server HELO string: xxxxxxxxxx.xxxxxxx.com 2013-01-23 06:13:25 Message id: 2013-01-23 06:13:25 Message reference: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 2013-01-23 06:13:25 Sender: firstname.lastname@example.org 2013-01-23 06:13:25 Recipient: email@example.com 2013-01-23 06:13:26 SMTP Status: OK 2013-01-23 06:13:27 Delivery attempt #1 2013-01-23 06:13:27 Recipient server: 0.0.0.0 2013-01-23 06:13:27 Response: 421 4.4.0 [internal] no MXs for this domain could be reached at this time 2013-01-23 06:19:57 Delivery attempt #2 (final) 2013-01-23 06:19:57 Recipient server: 0.0.0.0 2013-01-23 06:19:57 Response: 421 4.4.0 [internal] no MXs for this domain could be reached at this time
I've replaced all of the identifying bits obviously, but just to make it clear, the bit about
Recipient server: 0.0.0.0 is what's actually in the log.