Marc Perkel wrote:
Markus Stumpf wrote:
On Mon, Nov 27, 2006 at 02:42:28PM -0800, Marc Perkel wrote:
What this says is that if you decide to have some limits that the
limits
should be set at 2 or more. Since Qmail doesn't have a configurable
limit then the MUST part applies.
...
Read on in the same section:
Although the capability to try multiple alternative addresses is
required,
Note REQUIRED.
So qmail MUST try multiple alternative addresses. And in effect IT DOES
THAT (if connection cannot be established).
specific installations may want to limit or disable the use
of alternative addresses.
I believe what it means here is that the capability is required but you
might want to put limits on it because of resources.
I believe what it means here is that the capability can be subjected to
policy restriction. For qmail, policy rules are:
- if a connection cannot be established, try next MX.
- if a connection succeeds, but server is unable to accept transaction,
don't try next MX.
Compliant.
The question of whether a sender should
attempt retries using the different addresses of a multihomed host
has been controversial. The main argument for using the multiple
addresses is that it maximizes the probability of timely delivery,
and indeed sometimes the probability of any delivery; the counter-
argument is that it may result in unnecessary resource use.
This is what lawyers cal dictum. It's a discussion. The reason for
trying all MX is timely delivery. The exception is resources. If for
example you are running your server on a dialup connection at 56k you
can make an exception because retries use up your limited bandwidth.
"The main argument ... is", so it's not the only argument. Another
argument may exists: so "if the main server is up, try only that server
because sending message to the final recipient is faster" is another
good argument. Counter-argument is "but if sysadmin screwed it's
configuration, misused protocol, activated brain-dead anti-spam
measures, it will not receive the message" ...
Note
that resource use is also strongly determined by the sending strategy
discussed in section 4.5.4.1.
Now look at "4.5.4.1 Sending Strategy"
The sender MUST delay retrying a particular destination after one
attempt has failed.
By destination this means host.
Why? "Particular destination" may be also a mailbox, a domain, an IP
address. Qmail "particular destination" is "domain".
That means you don't keep hammering the
same host over and over.
And if you send the message to 2nd host, 2nd host will then "hammer"
first host for you? In Italy we call that "mafia" ... :-)
But it does not mean that you don't try other
servers. If it did then there would be no point to multiple MX records.
If SMTP client can't connect to first MX, may be 2nd MX can. So qmail
send message to 2nd MX if connection cannot be established.
--
Tullio Andreatta
Disclaimer: "Please treat this email message in a reasonable way, or we
might get angry" ( http://www.goldmark.org/jeff/stupid-disclaimers )
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