SMTP error 540 vs 450 (and running a mail server without an MX record)

Jorgen Grahn wrote:

I only speak for my self, but isn't SMTP fairly on-topic here,
in comp.mail.headers?

I want to discuss the following SMTP error message:

Is there much to say about it? 5.4.0 is the only information
you have (you can look up its meaning; if it is worth trying
again later and so on). The text just says this is an "other"
failure.

Ok, here's what I want to talk about with respect to error 5.4.0. You
will see how my question relates to SMTP in general.

For the past 2 years, I've been running our mail server without an MX
record. This originally happened when we moved our office and changed
ISP's. The missing MX record wasn't noticed for a few months -
because our A record pointed to the same IP as our MX record would
have if it was configured.

I have left our MX record in that (null) state because what did happen
was an immediate reduction in spam - I would guess 75% right off the
bat, and basically no growth in spam ever since. (I'm talking about
zombie-spam).

At the time, it seemed that the explanation was that a sending MTA
would fall back to a domain's A record upon an MX-lookup failure. We
encountered no external hosts (or people) who claimed they were
getting errors trying to send us e-mail. We are in touch with a
diverse set of educational and biomedical entities all over the world,
and we were able to maintain e-mail contact with them.

I recently became aware of a case where the lack of an MX record has
prevented proper communication.

Here is the error message as seen by the sender:

----------------
These recipients of your message have been processed by the mail
server: munged@xxxxxxxxxx; Failed; 5.4.0 (other or undefined
network or routing status)

Reporting-MTA: dns; (a server in Germany)
Arrival-Date: Fri, 5 Oct 2007 15:09:32 +0200

Final-Recipient: rfc822; munged@xxxxxxxxxx
Action: Failed
Status: 5.4.0 (other or undefined network or routing status)"
-------------------

I'm wondering why the error wasn't 450 (or 4.5.0 ???) instead of
5.4.0.

I'm also wondering if I should engage the postmaster of the sending
MTA and ask him why his server is not falling back to the A record.

Our organization really enjoys the very low spam load we currently
receive, and I would hate to activate our MX record without some idea
that we perhaps are missing quite a lot of incoming e-mail.

Comments?
.

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