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Re: Weird DNS issues for domains

  • From: Crist Clark
  • Date: Fri Sep 30 12:43:15 2005

Peter wrote:
Crist Clark <crist.clark@globalstar.com> wrote:
[...]

The problem I've seen is when an SMTP server does not accept emails
which have non-resolvable MAIL FROM domain. When the sender is a
dumb SMTP client, not an MTA, this can cause problems.

Well, that "dumb SMTP client" should stop pretending to be a MTA then.
If it can't queue and retry, it shouldn't even *think* about looking
for MX records.
Sorry, I guess I was not clear. The dumb client is not pretending
to be an MTA. The dumb client is sending to its "smart host." The
MTA, the smart server for the dumb clients, does a "reality check"
on the envelope sender. (This is not unusual.) A dumb client tries
to send,

	MAIL FROM:<joebillybob@down-dns.org>

Via the MTA, but the MTA rejects this because it cannot resolve the
domain. Now even if our MTA does the right thing and rejects with
a 4xx error, a dumb client may not be equipped to handle this well.

Besides, what sort of "dumb SMTP client" did you have in mind?
Formmail scripts? Worms? Outlook Express? I can't say I'd miss mail
from any of those.
Well, the reality check on the sender domain is meant to stop a lot
of traffic from some of those sources, so I won't miss that either.
However, due to the nature of our business, we have lots of people
with very, uh, "interesting" SMTP clients. I know of a few who have
integrated PPP/IP/TCP/SMTP stacks for custom hardware, i.e. they wrote
network code for a device with less CPU and RAM horsepower than your
modern wrist watch to only send email. They tend not to handle
exceptional conditions well (and sometimes have cool features like
the sender address is hardcoded, hardcoded in NVRAM, or hardcode the
IP address of the smart host which is fun when we move those or bring
one down for maintenance).

(I noticed this happen to a high traffic customer who had both of
their DNS servers in the same /24 located in Slidell, LA. Needless
to say, they were down for more than a few hours when Katrina rolled
through.)

Having reachable DNS isn't going to help anyway if the MX host is also
unreachable for an extended period. Mail is still going to bounce
after a few days if somebody doesn't fiddle with DNS.
But even if the destination MTA is reachable, the mail was not going
through since the MAIL FROM domain was unresolvable. The mail would
have been delivered promptly had the sender's DNS been available. The
sender's MX MTA never enters into the picture.
--
Crist J. Clark                               crist.clark@globalstar.com
Globalstar Communications                                (408) 933-4387




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