I'm trying to send email to a domain that has two MX records. The higher priority record is IPv4 and the 2nd is IPv6.
Exchange gives me a delivery is delayed message in my inbox after I've tried to send the email.
A look at the connectivity logs shows
Established connection to XXXXX (the correct IPv4 IP) Failed connection to XXXX (the correct IPv6 IP) Retry
Despite establishing the connection to the IPv4 address it doesn't start the message delivery. If I send the email via telnet from the transport server to the IPv4 address it is successful.
With other domains it would just deliver the email over the IPv4 IP. I'm sending to many other domains with a similar MX configuration without a problem.
My firewall is an ASA and my public IPs are IPv4. The transport server is Win 2008 x64.