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Subject: Re: msn.com & service denial
From: Brad Knowles (blkskynet.be)
Date: Wed Jun 07 2000 - 19:24:52 CDT

At 1:58 AM +0200 2000/6/8, Rask Ingemann Lambertsen wrote:

> Your assumption that the secondary MX just deliververs
> to the primary MX is very likely invalid for a large part of the email that
> goes to domains with more than one MX. Think of aol.com, hotmail.com,
> msn.com and the like.

        Speaking as the former Senior Internet Mail Systems Administrator
for AOL, I can tell you that this is *precisely* what happens there
-- the backup MXes are simply used as a holding pool until the
primaries come back up, and the mail can be delivered through.

        Contrariwise, I can tell you that both the primary and secondary
MXes that we have here will actually deliver directly to the
appropriate back-end server, although we would prefer that the vast
majority of that traffic pass through the primary MX.

        Getting back to the original question, technically the proper
behaviour would probably be to bounce a mail message if you get a 5xx
response, regardless of how it was generated or by which machine.

        Unfortunately, far too many machines generate 5xx responses in
situations that they should not, and we have been too "liberal in
what we accept" for too long, and therefore we cannot now decide to
become more conservative in what we accept. 

--
   These are my opinions -- not to be taken as official Skynet policy
======================================================================
Brad Knowles, <blkskynet.be>                || Belgacom Skynet SA/NV
Systems Architect, Mail/News/FTP/Proxy Admin || Rue Colonel Bourg, 124
Phone/Fax: +32-2-706.13.11/12.49             || B-1140 Brussels
http://www.skynet.be                         || Belgium