Looking at the header revealed hits like:

X-Spam-Status: No, score=10.994 tagged_above=2 required=12 tests=[AWL=-3.518, FH_DATE_PAST_20XX=10.357, FH_RELAY_NODNS=1.451, FM_FORGED_GMAIL=0.622, FRT_PROFILE2=1.981, HTML_MESSAGE=0.001, RDNS_NONE=0.1]

Thinking FH_DATE_PAST_20XX=10.357 looked a bit suspect, I did some Googling and found the following blog entry:

http://serverfault.com/questions/98723/workaround-spamassassin-y2k10-bug-mac-os-x-server

I went with Option 2 — modifying the rule configuration file such the 2010 isn’t perceived to be so far in the future as to be spam

Substitute

header FH_DATE_PAST_20XX Date =~ /20[1-9][0-9]/ [if-unset: 2006]

with

header FH_DATE_PAST_20XX Date =~ /20[2-9][0-9]/ [if-unset: 2006]

Unfortunately that didn’t resolve the issue for me.

On further searching through the various configuration files on the server, it looks like

/private/etc/mail/spamassassin/00_FVGT_File001.cf

is the file you want to edit.

[UPDATE:] yes, updating /private/etc/mail/spamassassin/00_FVGT_File001.cf and restarting the mail service appears to have resolved my issue.