I wonder if limiting the number of incoming messages per SMTP session
will have the effect you want - you would expect whatever mail
injection tool is being used would just start another session and
continue with scarcely any delay.
My own approach to the problem was to have a log processor which
processed the exim logs on the fly and rate limited on modem IP
addresses - it actually dropped information about spamming IPs and
message ids into a local database which was consulted by the system
filter.
Use of this would normally pick up all but the first few messages of a
spam run.
Nigel.
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[ Nigel Metheringham Nigel.Metheringham@??? ]
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