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Page 76: Message handling with MailScanner
MailScanner takes the second of the three options I laid out
earlier, i.e. it operates as a bump in the queue.
Several copies of MailScanner run symultaneously, a number chosen by
the sysadmin depending on the capacity of the machine. This is a
simple way of determining the maximum load that MailScanner can
impose -- a feature you can't get with SMTP-time filtering.
It processes messages in batches. When the load is light, the
batches are small so the delay to a message passing through the
system is also small. When the load is heavy, messages build up in
the incoming queue so the batches become bigger. However the
overhead of dealing with a batch is shared across more messages so
the cost per message actually goes down! This is the reverse of
SMTP-time filtering which makes the system thrash at high load.
MailScanner doesn't know anything about the complexties of Exim's
configuration and the subsequent message routing.
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