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Hello all,

In the past I have stated that having an MX record such as this:
example.com. MX 10 example.com.

would be redundant, because without it, the mail would be delivered to example.com anyway.

Well, it turns out there is a difference. Suppose you don't have the MX record, but only have this:
example.com. A 192.168.0.1

Then suppose someone sends mail to user@example.com. Now suppose that at the time of sending, the mail server is not responding. In this case, the sending (relaying) server assumes that, although there is an A record, there is no mail server at that address. Since there is no MX record to indicate intent of a mail server, the sending server concludes that there is no facility for accepting mail for user@example.com. The message bounces immediately.

Now repeat the scenario, but with the MX record. In this case, the sending server sees the MX record as a declaration of intent to receive mail. It waits a predefined period (generally 3 or 4 hours), then tries again, repeating until either the message is accepted by a listed server or it hits a predefined timeout (generally 1 to 7 days).

So there is indeed value in having an MX record, even if it points back to the name of the domain. (Of course, it's even better to have two MX records, one pointing back to a backup mail server.)

All of this was made clear to me earlier today when a message bounced, previous mail to the intended recipient having gone through. In this particular case, the machine at the server's address was pingable, but the mail server was down.
____________________________________________________________________
Chris Buxton cbuxton@menandmice.com
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