I am currently in the middle of migrating Exchange 2000 SP4 to Exchange 2007 SP1 with Update Rollup 9. When the Exchange 2007 installed it failed to create the send connector. I manually created this connector and since then we have started seeing some messages get stuck in the queue with the error:
451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." Attempted to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
I have the connector set up to use external and internal dns.
Researching this issue I found a forum post on MS Tech Net regarding DNS issues with IPv6 in Exchange 2007 sp1. There has been no clear answer on this and need to know if someone else has seen this and been able to rectify the problem. I need to clear this up before proceeding to Exchange 2010 which is part of the project.