I also agree with Ned, and maybe you, that explicitly documenting 550 for the set of "I don't like you, go away and don't come back" -- and maybe, for clarity, adding a 4yz code or comment about one for "I don't like you today, but, if you come back tomorrow, I might change my mind-- would clarify both the documentation and the situation.
I don't feel strongly about whether we need a separate 550 status, but I do think that if we agree that the correct response to a 554 is to fail and not try any alternative MX, it would be a good idea to say so.
Regards, John Levine, johnl at taugh.com, Taughannock Networks, Trumansburg NY "I dropped the toothpaste", said Tom, crestfallenly.
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