With the "sandwich" approach to content filtering described
here, it is important to match the filter concurrency to the
available CPU, memory and I/O resources. Too few content filter
processes and mail accumulates in the
active queue even with low
traffic volume; too much concurrency and Postfix ends up deferring
mail destined for the content filter because processes fail due to
insufficient resources.
Currently, content filter performance tuning is a process of
trial and error; analysis is handicapped because filtered and
unfiltered messages share the same queue. As mentioned in the
introduction of this document, content filtering with multiple
Postfix instances will be covered in a future version.