Regarding The external & internal DNS

[Kevin Darcy]

First of all, many mail systems don't even use DNS for routing. Some of
them have rule-based routing schemes, or directory-based routing schemes,
or whatever.

If they do use DNS for mail routing, however, then to answer your
question, no, MX records are not necessary in order to deliver mail
directly to a particular host. Read RFC 974 for details. If an MX record
doesn't exist for a particular host, then the mail server should do an
A record lookup and then use that for delivery.

Having said that, there are many reasons why you might want to use MX
records internally anyway:

1. For a centralized mail routing scheme. Internally here, for instance,
anyone can mail anyone else by using @chrysler.com addresses. This all
goes to a central set of mail relays which then forwards the mail to the
appropriate mailbox on the appropriate mail system. If someone changes
their mailbox or mail system, then the records just need to be changed on
the mail relays instead of in everybody's address books everywhere. So we
have a central point of routing control.

2. For performance. Since DNS-aware mail servers always do an MX record
lookup first, followed by an A record lookup if the MX wasn't found, it's
faster and more efficient to have the MX records there, even if they don't
technically perform a different function than the A records alone. Cuts
down on the number of lookups.

3. For indirect routing. Sometimes a host will be behind a gateway or
firewall for mail. In such a case, the MX record might point at the
firewall or gateway which is capable of routing mail to that host.

4. For failover and/or load-balancing. Based on preference values, MX
records can provide failover and/or load-balancing. The
"chrysler.com" servers, are load-balanced.


- Kevin

Ashish Kumar Batwara wrote:

> Hello,
> we are having two internal DNS server, one is primary & other one is
> secondary.
> In addition to that one internal mail server is also there. On top of
> that we are having firewall, which interacts with external network(DNS
> as well as mail server).
> If i am having some hosts in the internal network & they wants to
> send/receive mails to/from each other, then is it require that their MX
> record should be there in the internal DNS????
>
> Regards
> ---------
> Ashish
>
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