At 00:45 +0100 on 04/16/2008, Sabahattin Gucukoglu wrote about RFC2821bis-10 S5.1 Revision: "Unusable" MXs:
Hi all, I'm looking over RFC2821bis-10 now. Section 5.1 now has this (not sure if there's previous context from older drafts): | If MX records are present, but none of them are usable, or the implicit | MX is unusable, this situation MUST be reported as an error. What exactly does that mean? More specifically, what does "Usable" mean?
If the explicit MX or implicit MX (ie: Using existent A or AAAA [but not both since only one type exists]) that is defined for the email address's FQDN points at only IPv4 IPNs when the SMTP MTA only talks IPv6 (or a IPV4-only MTA finds only IPv6 IPNs) then you have an unusable condition since none of the IPNs found can be used by the MTA.
