Messages incorrectly getting quarantined

[Fugtruck]

I have a Postfix+Amavisd-new+Spamassassin setup that has been working fine for some time now.  Lately though I've had one particular sender having messages being quarantined as SPAM, despite the spam score being well below the sa_kill_level.  Here is the content analysis details from one of the messages.  Could anyone give me some pointers on where to begin troubleshooting this?  I am at bit of a loss as to what is causing this.
As a side note, I do have lots of mail successfully delivering from this server.  It just seems to be happening to this one sender (as far as I can tell).

Content analysis details:   (1.9 points, 5.7 required)

 pts rule name              description
---- ---------------------- --------------------------------------------------
 0.0 HTML_MESSAGE           BODY: HTML included in message
 1.2 MIME_HTML_ONLY         BODY: Message only has text/html MIME parts
 0.6 HTML_MIME_NO_HTML_TAG  HTML-only message, but there is no HTML tag
 0.1 RDNS_NONE              Delivered to internal network by a host with no rDNS
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