Great Circle Associates List-Managers
(December 1996)

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Subject: Re: Aggregating on MX records
From: "John Buckman" <jbuckman @ shelby . com>
Organization: Walter Shelby Group Ltd.
Date: Sun, 1 Dec 1996 21:01:08 +7
To: meo @ schoneal . com (Miles O'Neal)
Cc: dcrocker @ brandenburg . com, list-managers @ GreatCircle . COM
Comments: Authenticated sender is <jbuckman @ synergy>

> So what happens when a mailhost reroutes mail based on the domain? 
> For instance, webmaster @
 foo .
 com and webmaster @
 bar .
 com might be
> different addresses, where and are virtual domains
> on the same system.

It wouldn't change anything, since the same messages are
communicated as with domain-aggregation.  

Let's say and are both virtual domains at,
and that has mail server called and and have the same MX information, namely that the 
primary MX host is, and secondary host is

Now, let's say webmaster @
 foo .
 com and webmaster @
 bar .
 com both belong a 
a list called "test-l", and a single email message has been sent to 
that this.

With domain-based aggregate mailing, the mailer would see
webmaster @
 foo .
 com and webmaster @
 bar .
 com as unrelated email addresses, 
and make two separate DNS inquiries, and two separate SMTP sessions 
(and connections) in order to transmit those two messages.  

With MX-based aggregate mailing, what happens depends on whether the
two messages to send are identical, or different.  If they are two
different messages, then the SMTP sessions are the same as with
domain-based aggregation, except that the two messages are sent in
serial, in one TCP/IP session, rather than as two sessions, so you
gain something in starting up fewer TCP/IP connections.

If the two messages are identical, then MX-record aggregation really
improves things, because you can add an extra RCPT TO:<> to the first
message, for the 2nd recipient, and you'll have saved almost 1/2 the


Perhaps someone can answer this question: do DNS servers cache MX
records?  Obviously, most DNS servers cache hostname->IP lookups, but 
I was wondering if they cached MX entries as well.  

Or, are there any mailers (or patched mailers) which cache MX
records?  It seems like an obvious place to increase efficiency,
especially with busy mailing lists, so I'd be suprised if some mailer
(MTA) didn't do this.


john @
 shelby .
 com, Shelby Group Ltd.,

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