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When procmail is calling SpamAssassin and a large number of messages come in, SpamAssassin may occasionally fail to run, perhaps because the load exceeds the allowable quota. Procmail then recovers the mail and continues with the procmail script, but with the message having gotten no SpamAssassin markup whatsoever. This results in a false negative if the mail is spamspam getting through – without the "probably spam" markup from SpamAssassin. The solution (other than moving to a more powerful server, which also works) is to add a second SpamAssassin invocation to procmail in case the first one didn't work. Compare to the default script shown at UsedViaProcmail.
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