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Re: RFC2821bis-10 S5.1 Revision: "Unusable" MXs

2008-04-15 21:31:49

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

| 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.

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