From: Peter Rabbitson (rabbitrabbit.us)
Date: Tue May 01 2007 - 14:12:25 CDT

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Wietse Venema wrote:
> Peter Rabbitson:
>> Hi,
>>
>> I am experiencing 421 errors between my secondary and primary MXes, and
>> it seems it is cause by lack of connection caching.
>
> What is the error message?
>

There is no error message as such, see below.

>> http://www.postfix.org/postconf.5.html#smtp_connection_cache_on_demand
>> misses to explain what is "high volume of mail in the active queue".
>> When is exactly connection caching activated?
>
> Roughly, it is activated when the active queue contains another
> message before the current delivery is completed.
>

If the primary MX is down for an extended period of time and a large
queue accumulates on the backup, all messages are rushed to the primary
MX in what it seems separate smtp connections. At least I was able to
count as many smtp processes in `ps` as 2/3 of the number of queued
messages, right after I issue `postfix flush`. If I specify explicit
caching for the particular mx host, things work as expected. I guess
there is not enough time for the caching on demand to activate when
doing a flush or having enough queued messages to simulate one.

Peter

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