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(?) Domain Renaming and E-mail Routing and Re-writing

From Peter Stilling

Answered By Jim Dennis

Mr. Dennis,

Currently our campus is making a domain change from ricks.edu to byui.edu We would like all of our email that is addressed to ricks.edu to be forwarded to the new byui.edu domain. Is there a way to do this with MX records some how?

Peter Stilling

(!) [JimD] Changing domain names is hard.
This cannot be done with MX records (alone). It must be done by the MTAs (sendmail, qmail, postfix, exim, or whatever you want to use). The MX records will associate a list of preferences and destinations with an e-mail domain. Usually all but one of the MX destinations will be relays (your ISP, a couple of your well-connected and reasonably trustworthy friends or "partners"). Those will all contain higher precedence values (meaning "less preferred") so any proper SMTP MTA which attempts to deliver mail to that domain will ignore all the secondary/tertiary MX hosts and attempt to contact the (usually one) with the lowest precedence. (The others are for "fallback" when the preferred destination is unreachable).
So with MX records you can say that mail to ricks.edu should be delivered to a machine (hostname) which is in the byui.edu domain. Note that you MUST use a hostname and not a "CNAME" or alias and not an IP address. Of course the hostname must be listed in some valid DNS zone which provides one or more A (address) records for it.
Let's say that you choose to deliver all ricks.edu mail to rexburgmx.byui.edu. Publishing a set of MX records like:
ricks.edu.	IN MX 10	rexburgmx.byui.edu.
		IN MX 20	mx.byui.edu.
		IN MX 30	mail.backbone.not.

rexburg.byui.edu. IN MX 10	rexburgmx.byui.edu.

rexburgmx.byui.edu.  IN A
... would serve to get mail delivered to the machine at
(Any host that couldn't reach that machine would try to drop it on
mx.byui.edu and thence on mail.backbone.not (Note the bogus top-level domain here --- it's for example only!). Those secondary MX destinations should be configured to relay mail to your primary host. (It used to be allowed by default in sendmail and most other MTAs --- however the spammers exploited this courtesy and laissez faire approach and have increased the burden on sysadmins and postmasters everywhere).
Now the host named rexburgmx.byui.edu would have to be configured to consider itself the mail destination for the ricks.edu mail domain. In sendmail terms we'd have to add ricks.edu to the "who am I" class (Cw) or file/list (Fw). In qmail we'd put this in the locals or the me control file. In Postfix we'd add it do the "mydestination" list (or keyed/database file).
Unfortunately that approach, by itself might lead to some oddities. Outgoing mail from this system might end up with headers and envelope "From" addresses set to the byui.edu domain, or some rexburg.byui.edu subdomain. (In the worst case they'd end up with the hostname as their from address). In sendmail terms they might be "masqueraded as" being from byui.edu.
Here's where we get complicated. You have to make some policy decisions about how you want mail headers to look at mail goes out of your domain. This will effect how replies get routed back to you. There is no "right way" to do it. There are many possibilities and pros and cons to each.
Perhaps all you want is a simple transitional delivery mechanism. Perhaps all new addresses will be in the rexburg.byui.edu domain or even directly in the byui.edu domain or in various departmental subdomains under byui.edu --- sci. (or science students and faculty), cs (computer science), adm, admin or staff (for administrative staff) etc. Perhaps you intend to move all the old accounts and e-mail addresses to new ones (or you've already done so and resolves any name collisions that arose).
In those cases the ricks.edu e-mail domain exists purely so that mail to the old addresses gets delivered to the proper recipients. All responses to foo@ricks.edu can reasonably have a From: address of foo@byui.edu or possibly even foo_bar@byui.edu (where the mail was re-written through some form of aliasing, perhaps to resolve a name collision between Mr. Foo at byui.edu and Ms. Frances Oo (no relation) at ricks.edu).
Thost are the sorts of things that make this sort of transition "interesting." Corporate mergers and aquisitions make it a fairly common occurence; which doesn't make it any easier. Unfortunately I can't describe a simple procedure for you to follow. There are too many variables.
As implied by my title, here: renaming your e-mail domain has two distinct aspects; routing the incoming mail to its recipients (mailboxes) and generating/re-writing headers on outbound mail so that responses can make it back to their authors.
Have fun. You're in for a learning experience.

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