Enabling IPv6 on a Mail Server

From ARIN IPv6 Wiki

You need to think through all the interactions before enabling IPv6 on a mail server.

Ket Crispin posted this to the IETF list:

  I was presenting what I thought was an interesting example
  of a subtle problem that can come up in ipv6 deployment.  
  
  The mailserver in question uses a default redhat enterprise 
  build (actually centos).  ipv6 is either enabled by default, 
  or just has a single check box, with no further information.
  The fact that ipv6 is enabled so trivially carries the implication 
  that just enabling ipv6 won't actually damage anything. 
  
  Now I know different.  Just enabling ipv6 on an otherwise 
  correctly configured and functioning ipv4 box *will* cause 
  damage -- it will cause mail that would have been delivered 
  to not be delivered.  I could be wrong, but this strikes me 
  as a trap that lots of people could fall into. 
  
  As I mentioned, my servers actually do reject mail if they 
  can't find a reverse dns for the senders IP. Some of those 
  servers use ipv6; in light of all this I'm going to have 
  to rethink that decision.  For a server, the combination of 
  enabling ipv6 and using this particular anti-spam technique may 
  drastically increase the number of false positives -- especially
  as ipv6 gets more widely deployed.