MX records and multiple IP addresses

[Jim Reid]

>>>>> "Richard" == Keller, Richard (CIT) <keller at mail.nih.gov> writes:

    Richard> We have a client who is requesting the following: They
    Richard> currently have an email server with an MX record pointing
    Richard> back to it.  So let's say the record is
    Richard> mailserver.nih.gov. IN MX 10 mailserver.nih.gov.  They
    Richard> wish to have another email server, mailserver2.nih.gov,
    Richard> that will be on the same IP address, but will be used for
    Richard> IIS web access to exchange.

    Richard> If they are registering mailserver2.nih.gov to the same
    Richard> IP address as mailserver.nih.gov will it need another MX
    Richard> record that points to mailserver2?  In other words,
    Richard> mailserver2.nih.gov. IN MX 10 mailserver2.nih.gov.  Or,
    Richard> should the MX record point back to mailserver.nih.gov.

It probably doesn't matter. The important thing is that mail addressed
to mailserver2.nih.gov gets dropped on an IP address that knows to do
the Right Thing with that mail.

An MX record like
	foo.bar. MX 10  foobar.com.
means "deliver mail addressed to somebody at foo.bar by speaking SMTP to
foobar.com". foobar.com should be an A record, not a CNAME. That's it.
So an MX record of mailserver2.nih.gov. IN MX 10 mailserver2.nih.gov
is just fine as long as there's an A record for mailserver2.nih.gov
and the mail server at that IP address can deal with mail addresses of
the form username at mailserver2.nih.gov.

Where you can expect to have some fun is if mailserver.nih.gov and
mailserver2.nih.gov are different systems and your client wants them
both to be on the same IP address. This seems to be what you're
suggesting, though maybe you've phrased the problem rather poorly.

    Richard> In addition, will multiple MX records on the same IP
    Richard> address cause any kind of problems routing e-mail or with
    Richard> name resolution?

Probably not for inbound mail. All you need to do is configure
sendmail or whatever on your mail hub to figure out which mail
disappears into Exchange and which stuff gets routed elsewhere
internally. You also have to be careful about what that mail system
does to the headers on outbound mail.