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.
This document technically contradicts itself. Not written by a lawyer.
As you can see in one place it says MUST and then gives an exception.
Or it says REQUIRED but then another exception. So lets see what it
really means
Read on in the same section:
Although the capability to try multiple alternative addresses is
required,
Note REQUIRED.
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.
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.
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. That means you don't keep hammering the
same host over and over. 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.
In general, the retry interval SHOULD be at
least 30 minutes; however, more sophisticated and variable strategies
will be beneficial when the SMTP client can determine the reason for
non-delivery.
Exim has a nice timing feature where after a number of hours the retry
interval gets lomger.