We have a pretty standard config on our Exchange Server, static IPv4 address, etc.
We relay all outbound emails through the AntiSpam server.
All of a sudden today everyone was getting Relay Access Denied NDR's, and after speaking with the AntiSpam server provider it seems the reason is because the server is all of a sudden using IPv6 in the headers (which are not in the ACL)
Now this in itself is an inherent problem becuase IPv6 although enabled on the Exchange Server, is not configured.
So I can workaround this issue by adding the IPv6 address into the AntiSpam box as "allowed to relay" but as IPv6 is set to DHCP, it could change?
Ideal solution will be to force it to use IPv4 (but I dont want to remove IPv6 as I have known this to cause issues on Exchange)