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.