maillscanner/postfix saturates bandwidth :-(

C. Jon Larsen jlarsen at
Mon Sep 24 20:09:33 IST 2012

On Mon, 24 Sep 2012, Harondel J. Sibble wrote:

> Had an odd situation that started Friday night at one of my clients running a
> mailscanner/mailwatch mail relay for their internal Exchange 2007 server.
> Basically the dsl connection they share with another office was saturated
> when the office admin did a mailout on friday to about 2000 of their
> subscribers, each email was about 3.5mb total with conversion overhead.  When
> I say saturated, I mean in both the upstream and downstream directions.
> According the admin who runs the multitenant network in this office, he was
> seeing a sustained 1.6mb/s INBOUND connection to my client's firewall while
> this was happening.
> I intially though that someone had hacked in and was injecting spam, but
> after upstream throttling of the connection and disabling all smtp traffic, I
> was able to review the messages in the postfix deferred queue and determine
> they were part of the mailout.
> At this point mailq was showing 14 messages with approx 100 recipients total,
> I could then re-enable smtp traffic (in/out) at the firewall level and emails
> would be fine sending and receiving, but if I did a postqueue -f, the
> connection would saturate again until I blocked the smtp traffic, then waited
> a couple minutes before re-enabling it and the messages went back to being
> deferred.
> I'm trying to figure out the best way to deal witih this moving forward, is
> there additional throttling I need to do at the postifx level or the
> mailscanner level or something else.  I was also surprised as my understand
> of postfix is that it does connection throttling by default.

You can play with variations of these settings in main.conf to control how 
much email is sent out - these go into main.conf

local_destination_concurrency_limit = 2
default_destination_concurrency_limit = 2
initial_destination_concurrency = 2

smtpd_client_connection_count_limit = 10
default_destination_recipient_limit = 20

> -- 
> Harondel J. Sibble
> Sibble Computer Consulting
> Creating Solutions for the small and medium business computer user.
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