Hi Michael,
At 10:04 14-07-2005, Michael Kenny wrote:
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."
The SMTP service was not up. The server should have retried delivery
to the other relay servers according to your description in paragraph
one. This relay strategy is a local implementation by the vendor and
is not covered by RFC 2821.
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?
The client should rely on the SMTP response. It should retry the
next server in line if that implementation is like MX records.
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"
This is discussed in RFC 2821. See the "Sending Strategy"
section. If you connect to a SMTP service and you don't get an
initial reply (Section 4.5.3.2), you consider that service as down.
Regards,
-sm