DNS Cache and Sendmail Msg-Id

Julian Field mailscanner at ecs.soton.ac.uk
Tue May 20 19:38:42 IST 2003

At 19:17 20/05/2003, you wrote:

>Sorry, now that I look at it a little closer, I'm thinking of the ESMTP id
>as listed in the received headers.

I think the answer you are looking for is "60 years".
Old versions of sendmail used a primitive id (i.e. what is used as the id
in the filename in the queue), which would repeat. So they changed it to
use a system that would be unique over a period of 60 years (which has to
be enough, surely :-)

>-----Original Message-----
>From: mikea [<mailto:mikea at mikea.ath.cx>mailto:mikea at mikea.ath.cx]
>Sent: Tuesday, May 20, 2003 1:58 PM
>To: MAILSCANNER at jiscmail.ac.uk
>Subject: Re: DNS Cache and Sendmail Msg-Id
>On Tue, May 20, 2003 at 01:31:25PM -0400, Derek Winkler wrote:
> >
> > Couple of kinda off topic questions...
> >
> > Is setting up a caching DNS server for the MailScanner machine a good
> idea?
> > Not sure how this would effect RBL lookups.
> >
> > How unique is the sendmail message id, is it unique for that day or until
> > the process ids rollover or some other method used? Searched the internet
> > for this but couldn't find anything.
>Done properly, the caching DNS server will retain records for their
>TTL, so that you won't have to go all the way to DNSbl server each
>time you look up, e.g., flowgo.com. Unless, that is, I'm much
>The sendmail message ID is intended to uniquely identify the message
>in the universe of all possible messages sent by all sendmail MTAs.
>It may even succeed at that. Certainly the date and time, sendmail
>process ID, and so on, are encoded in the 8.12 Message-ID -- down to
>a very small quantum of time. IIRC, earlier sendmails used a similar
>method, but the quantum was larger. Again, that's the way I understand
>it, but ICBW.
>Here's a randomly-chosen Message-ID from my system:
>Message-Id: <200305201731.h4KHVapE058947 at mikea.ath.cx>
>              <date-->time moretime<pid-> machinename
>Mike Andrews
>mikea at mikea.ath.cx
>Tired old sysadmin since 1964

Julian Field
Professional Support Services at www.MailScanner.biz
MailScanner thanks transtec Computers for their support
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