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Mystery Question

From Robert Polk on Thu, 20 Jan 2000

Hello,

Currently I am contracting for an LA based company. We currently have internal mail as well as a majority of users still using POP mail from an ISP.

We'll say the domain name is z.com. My question is in regard to MX records.

Example: domain z.com

 MX preference 10 external.z.com
 MX preference 10 mail.z.com

then it continues with 20, etc preferences.

Currently most users are still using this external mail for their internet email (external.z.com). There are however, about 20 users internally that are only using the internal mail host for their internet email (mail.z.com)=

I'm not sure what the question is.

There is a problem if you're using both of these hosts (mail.z.com and external.z.com) as POP servers for the one domain. Unless you're doing some sort of address rewriting or other mail routing on one or the other of these then any piece of mail for z.com could land on either of these hosts. Since each of them has equal MX values, and there are no lower MX values for the z.com domain, the mail that lands on one or the other will stay there.

In general you don't want to have this ambiguity. The lowest MX for a given domain or subdomain should lead to one unique host. The secondaries and tertiaries can be ambiguous since they will all relay their mail to the lower MX hosts.

You can do scaling in a few ways at the MTA level. For example you can have a big aliases file (DBM) or a set of NIS/NIS+ maps or an LDAP server to provide aliasing of recipients to specific POP servers. This can give provide a "flat" external mail profile (everyone is @z.com) while allowing you to scale to a much larger number of users than you could support on a single POP server.

(This assumes also that you'll configure all the mail hubs to re-write outgoing mail to masquerade the From: and Return: addresses).

Maybe the question you were asking is how to have your mail servers distinquish between the internal and external users. In that case, just use aliases. Mail that's addressed to "external.z.com" will ignore the z.com MX record. Likewise for any that's addressed to "mail.z.com"

To understand this you must note the distinction between where hosts to which mail is SENT and those to which it is ADDRESSED. Mail is sent to a host according to it's MX records. What that host does with it depends on how it is addressed. If the mail is addressed to any of the names that are on on a hosts "whoami" list (the sendmail 'w' class) then rewriting, aliasing, and local delivery are performed.

(I'm shamelessly limiting the MTA terminology here to that of 'sendmail' --- the various other MTAs have similar features and concepts).

Anyway, I've said more than enough given that I have no idea of what your question really is.

Copyright © 2000, James T. Dennis
Published in The Linux Gazette Issue 50 February 2000

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