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Article: 312
From: Anthony Howe
Date: 2005-01-25 02:40:50 -0500
Subject: Re: milter-ahead on a backup mx

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Anthony Howe wrote:

> Erik Hensema wrote:

>>Now I want to install milter-ahead on our backup MX, but I was thinking to
>>alter the behaviour a bit: when the backup MX succesfully makes an SMTP
>>connection to the downstream server, the backup MX should reject the
>>connection with a 4xx temporary error. This is because when the backup MX
>>can make a connection to the primary, the client should also be able to do
>>so.
> 
> Interesting twist. You are right of course that if the primary is up 
> that by definition of how MX servers are to be used that the client 
> should be contacting them always unless. Most spammers abuse secondary 
> MXes in an effort to slip past spam filters.
> 
> My only concern is some primary MXes temporary reject a message, using 
> grey-listing, or simply busy. Some poorly written clients then try the 
> secondary immediately (most likely spamware) or queue the message to 
> retry later on the secondary.

I've been rereading RFC 974 MAIL ROUTING AND THE DOMAIN SYSTEM. In 
particular section "Interpreting the List of MX RRs", paragraph 7, 
sentence 2 and 3:

	The mailer is required to attempt delivery to the lowest
	valued MX. Implementors are encouraged to write mailers so
	that they try the MXs in order until one of the MXs accepts
	the message, or all the MXs have been tried.

The only requirement above is a client MUST try the primary MXes
first before trying secondary MXes. It does NOT say that they MUST only 
delivery to the primary when it is online. The above suggestion might 
break legit mail delivery in the event of temporary (421 busy signal) or 
permanent rejection (554 weclome message) from the primary.

I'm willing to entertain an option to try this, but I have a feeling it 
will ultiimately be a bad idea. Comments?

-- 
Anthony C Howe                                 +33 6 11 89 73 78
http://www.snert.com/       ICQ:
7116561         AIM: Sir Wumpus

            "Once...we were here."  - Last of The Mohicans


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