[Spambayes] better Received header tokens

[Skip Montanaro]

    Alex> Alternately, we could drop the limit on the number of tokens
    Alex> looked at from 150 back down to around 20...

I look at all those tokens as many different ways for a message to exonerate
or incriminate itself.  If the various meta-tokens provide five (just to
pick a number out of thin air) more-or-less independent ways to say, "this
looks like spam", it's less likely that a spammer will successfully figure
out how to circumvent all five schemes.  The only positive effect I can
imagine is improved performance of the classifier, which would generally be
drowned out by either Python startup costs or networking overhead.

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