I put together a new SpamAssassin rule that will help identify spam from spoofed email addresses.

Some email providers always sign email with DKIM or DomainKeys … based on this assumption, if you get a message from one of those domains and it isn’t signed, you can assume its more likely to be spam.

This particular rule operates on the assumption that all mail from Yahoo & Gmail will be signed.  It does not, however, raise the score a huge amount … because it’s just more LIKELY to be spam if it’s not signed … it’s not guaranteed to be spam. Some people may use the Yahoo or Gmail account’s in the from address, but not actually send from that service.
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