Agreed! Spamassassin behavior seems strange on dkim signatures. Below is an example from one message and this is the scoring by SA
- Gain 0.1 score because message has at least one valid dkim sig
- Lose 0.1 score because message has a dkim sig (whether its valid or not)
Despite this apparent contradiction of no net gain in spam score from using dkim sigs, there is a further gain of 0.1 score if the dkim sig comes from the original author’s domain.
So long story short, DKIM sigs do help spam score but only marginally.
X-Spam-Report:
* -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP
* 0.0 HTML_MESSAGE BODY: HTML included in message
* -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature
* 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily
* valid
* -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from
* author’s domain