I don't have a precise solution for you, but I can offer these observations. For your IEA machine, Postfix MTA is probably the MTA that is doing the mail delivery. I can believe it is following RFC standard but falling back on the A record. I was able to get the same results that you got.
RFC 2821 section "5. Address Resolution and Mail Handling" goes over the MX and A record lookup. http://www.ietf.org/rfc/rfc2821.txt
"If no MX records are found, but an A RR is found, the A RR is
treated as if it was associated with an implicit MX RR, with a
preference of 0, pointing to that host."
I tried to look up how you can force Postfix to not fall back on the A record entry, but was unsuccessful on getting it to work. I think if you're going to get it to not use the A record and instead just get deferred if no MX record exist, it would be in the realm of Postfix, as this is probably the MTA doing the delivery.
I tried to use this setting called "ignore_mx_lookup_error=no", restarted Postfix, but it still used the A record. Hope that helps.
/etc/postfix/main.cf
ignore_mx_lookup_error = no