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Definition of server connection failure?

2005-07-14 10:04:40

I'm a network/email admin who just joined the list. I've reviewed the
archives as best I could, and read what I think is the relevant RFC
(2821) and I'm still a little unclear on something. I'm hoping you
guys can help me clear this up, and at the same time, that I'm not
asking something obvious and thereby making myself look really stupid
with my first post on the list.

We use the Tumbleweed EMF as our outward facing mail relay. It does
anti-virus scanning and spam filtering for us and then relays to one
of three Domino R5 servers for further inbound delivery. The
Tumbleweed server maintains an internal list of the inbound mail relay
servers to try, in order, as listed in their Relays options setup.

Due to a problem on one Domino server (the first in the Tumbleweed
relay list,) we shut down the Domino service on that box. The OS
(Win2K Advanced Server) was still running. No other service is
listening on the SMTP port.

The problem surfaced when one Tumbleweed server tried to deliver mail
inbound to the downed Domino server. When it could not, the Tumbleweed
EMF server generated a delivery failure and sent it back. It never
tried the other two servers in it's relay list.

The answer from Tumbleweed was this: "The EMF server detected the
server was up, so it never retried the others."

That strikes me as extremely lame. There was not , nor could there be,
any sort of SMTP response to the delivery attempts. I don't know what
detection method they are using to determine the Domino box was up
(ping? IP connection to the port? I don't know.)

RFC 2821 says, first sentence in section 3.1: "An SMTP session is
initiated when a client opens a connection to a server and the server
responds with an opening message."

I could find no other information about what constitues a connection
failure. Perhaps it's so obvious as to not even be asked.

But is it so that if there is no correct reponse to an SMTP session
initation, then there is no session and that SMTP server cannot be
considered "Up" and the next server in line should be tried? Normally
next is determine via MX records and routing cost, I know, but in this
case the next in line is determined in their internal list.

Two questions: Is that assumption correct? That is, should an SMTP
client rely solely on correct SMTP responses and not some other
machine state to determine if a server is up and whether to try the
next server in line?

And: Is delivery failure definition and the resulting behavior clearly
outlined in an RFC somewhere? I've seen the rules about how to handle
MX record requests and how to order them and the resulting deliveries.
But I guess I missed something that clearly defines when an SMTP
server is supposed to be considered "Down"

Thanks for taking the time to read this and I hope I've been clear and
not asked something moronic.



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