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Tip: examine exactly which rules were triggered. Take careful note of the points listed beside each rule name – low-scoring rules do not make much of a difference, ; it's the ones with high scores that need to be avoided.
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If you admin a SpamAssassin installation and suspect something in the configuration may be wrong, see here: AvoidingFpsForAdmins
If you are the recipient, simply add that sender to your ManualWhitelist. (You could also change the score of the rules that were triggered. See WritingRules).