On Mon, 30 Jul 2012 10:07:37 -1000, William Herrin said:
If you can reference where in the SMTP RFC it offers an authoritative explanation what to do when merging results from various naming systems where one but not all of the naming systems has generated an error then let's read it.
RFC5321, section 5.1 is pretty clear on it: 5.1. Locating the Target Host Once an SMTP client lexically identifies a domain to which mail will be delivered for processing (as described in Sections 2.3.5 and 3.6), a DNS lookup MUST be performed to resolve the domain name (RFC 1035 [2]). The names are expected to be fully-qualified domain names (FQDNs): mechanisms for inferring FQDNs from partial names or local aliases are outside of this specification. The Internet uses DNS. You use some other scheme at your own peril, and probably shouldn't expect said other scheme to work outside the range of your administrative control.