> And by the way, what is an MX? <g>
.Ah "How a client finds a server"
An SMTP client can choose an SMTP server several ways. The most
common way is that the SMTP client found delivery information in the
domain name system (DNS -- http://www.crynwr.com/rfc1035/). An SMTP
client can look up the mail exchanger (MX) record for a hostname, and
deliver the mail to one of the hosts listed in that record. The line
in the zone file for example.com would look like this:
example.com. IN MX 0 mail.crynwr.com.
This is an MX record with a priority of zero (the highest possible).
A hostname can have multiple MX records, each with a specified
priority. If you use the same priority, the SMTP client should choose
one at random.
Caution: the right-hand-side of an MX record MUST be a hostname. Some
versions of BIND (aka named) allow you to put an IP address, however
because it is not allowed in the DNS standard, you cannot rely on all
SMTP clients being able to parse it.
If there is no MX record (and it is optional to have one), the SMTP
client would look up an address (A) record for the hostname. If it
finds one, the client connects to the SMTP server at that address.
A client might also choose a server by using it as a "smarthost".
Many, many client workstations do not keep their own email queues, but
instead relay email to a server. This smarthost then routes the mail
to its destination SMTP server, whether locally or remotely. There's
nothing in the DNS to designate a server as a smarthost. It must be
configured by hand.
Yet another method of locating the right server is to use information
from outside the DNS. This would be unusual, since the DNS is usually
correct. The client might be working around an error in the DNS, or a
software incompatibility, or mere whim. If you find yourself needing
to do this for qmail's SMTP client (qmail-remote), you would insert a
record into control/smtproutes.
--
-russ nelson <[EMAIL PROTECTED]> http://crynwr.com/~nelson
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