Problem
Some of your incoming emails are getting tagged as spam by spamassassin.
Environment
In the email headers you will see the checks spamassassin performed to tag the emails. By default a score of 5 or above will mark the email as spam.
X-Spam-Flag: YES
X-Spam-Score: 5.749
X-Spam-Level: *****
X-Spam-Status: Yes, score=5.749 tagged_above=-1000 required=5
tests=[HTML_MESSAGE=0.001, LOCALPART_IN_SUBJECT=0.73,
RDNS_NONE=1.274,
TO_NAME_SUBJ_NO_RDNS=2.999, URIBL_BLOCKED=0.001,
URIBL_SBL=0.644,
URIBL_SBL_A=0.1] autolearn=no autolearn_force=no
The most scoring tests are TO_NAME_SUBJ_NO_RDNS and RDNS_NONE which is sort of double check as both tests check for DNS resolution.
Solution
Add your sender’s email address to the whitelist in /etc/spamassassin/local.cf:
whitelist_from sender@source.dom