SpamAssassin

For those of you who are still masochistic enough to run your own mail servers, here’s what mine looks like now, with some commentary on my discoveries:
# to test: spamassassin --lint
# test msg: spamassassin -t < MSG
# rule desc: grep -h "^describe.*XYZ" /var/lib/spamassassin/*/*/*.cf

time_limit 300
report_safe 0
add_header all Report _REPORT_

# This seems to do nothing:
report_wrap_width 255

# This improves X-Spam-Status but makes X-Spam-Report unreadable:
# fold_headers 0

# Don't query these guys:
dns_query_restriction deny zen.spamhaus.org     # Many false positives
dns_query_restriction deny iadb.isipp.com       # Many false negatives
dns_query_restriction deny list.dnswl.org       # Many false negatives
dns_query_restriction deny multi.uribl.com      # Blocked me?
dns_query_restriction deny list.dnswl.org       # Blocked me?

# Technical
score MISSING_MID               5       # Missing Message-Id: header
score MISSING_DATE              5       # Missing Date: header
score SPF_NONE                  2       # sender doesn't publish SPF
score SPF_SOFTFAIL              2       # sender doesn't match (softfail)
score SPF_FAIL                  3       # sender doesn't match (fail)
score DMARC_MISSING             2       # Missing DMARC policy
score DMARC_NONE                2       # DMARC none policy
score DMARC_REJECT              4       # DMARC reject policy
score DMARC_QUAR                4       # DMARC quarantine policy
score DKIM_ADSP_CUSTOM_MED      2       # No sig, adsp_override is CUSTOM_MED
score DKIM_ADSP_DISCARD         4       # No sig, domain suggests discarding
score FORGED_GMAIL_RCVD         3       # From gmail does not match 'Received'
score SPOOF_GMAIL_MID           2       # Not really from Gmail
score FREEMAIL_FORGED_REPLYTO   4       # Freemail in Reply-To, but not From
score FREEMAIL_FORGED_FROMDOMAIN 3      # 2nd level in From and Envelope differ
score SPOOFED_FREEMAIL_NO_RDNS  2       # From SPOOFED_FREEMAIL and no rDNS
score SPOOFED_FREEM_REPTO       2       # Forged freemail w/ freemail reply-to
score MISSING_HEADERS           4       # Missing To: header
score FROM_MISSP_EH_MATCH       3       # From misspaced, matches envelope
score PDS_FRNOM_TODOM_NAKED_TO  4       # Naked to From name equals to Domain
score DATE_IN_FUTURE_Q_PLUS     2       # Date: 4 months after Received: date
score FROM_FMBLA_NEWDOM         4       # domain registered in last 7 days
score FROM_FMBLA_NEWDOM14       3       # domain registered in last 14 days
score FROM_FMBLA_NEWDOM28       3       # domain registered in last 28 days
score HEADER_FROM_DIFFERENT_DOMAINS 1   # From/Envelope 2nd level differ
score FROM_IN_TO_AND_SUBJ       2       # From address is in To and Subject
score FREEMAIL_REPLYTO          3       # Reply-To/From differ
score HTML_FONT_LOW_CONTRAST    1       # HTML color identical to background

# Content
score HK_NAME_MR_MRS            4       # I am Mr.
score FORM_FRAUD_5              3       # Fill a form and many fraud phrases
score ADVANCE_FEE_4_NEW         3       # Nigerian 419
score NA_DOLLARS                2       # Talks about a million dollars
score MONEY_FREEMAIL_REPTO      3       # Lots of money from free email
score LOTS_OF_MONEY             1       # Huge... sums of money
score ADVANCE_FEE_3_NEW_MONEY   3       # Advance Fee fraud and lots of money
score URG_BIZ                   3       # Contains urgent matter
score HOSTED_IMG_DIRECT_MX      4       # Image CDN, message direct-to-mx
score FSL_BULK_SIG              2       # Bulk signature with no Unsubscribe
score FROM_EXCESS_BASE64        2       # From: base64 encoded unnecessarily
score BITCOIN_TOEQFM            4       # Bitcoin + To same as From
score PRICES_ARE_AFFORDABLE     2       # Prices aren't too expensive
score T_PDS_FROM_2_EMAILS       2       # From header has multiple addresses
score BAD_CREDIT                2       # Eliminate Bad Credit
score NUMERIC_HTTP_ADDR         3       # Numeric IP address in URL
score FREEMAIL_REPLY            3       # From/body different freemails
score URI_WP_HACKED             3       # Compromised WordPress site
score UNDISC_FREEM              3       # Undisclosed recipients + freemail

# Reject mail that is obviously in a language other than English.
# This is supposed to be a one liner, but instead it is this big
# mess because none of this works as documented.
#
if plugin(Mail::SpamAssassin::Plugin::TextCat)
  # add an X-Spam-Language header
  add_header all Language _LANGUAGES_

  # Nope: TextCat doc says this will fire but it does not:
  #  ok_languages en
  #  score UNWANTED_LANGUAGE_BODY 3

  # Nope: ok_locales doc says these will fire but they do not:
  #  ok_locales en
  #  score CHARSET_FARAWAY 3
  #  score CHARSET_FARAWAY_BODY 3
  #  score CHARSET_FARAWAY_HEADERS 3

  # This works, but only not when run under Postfix, not by "spamassassin -t".
  #
  score    __HAS_LANGUAGE_1 0.01
  describe __HAS_LANGUAGE_1 Language was identified
  header   __HAS_LANGUAGE_1 X-Languages =~ /../
  score    __BAD_LANGUAGE_1 0.01
  describe __BAD_LANGUAGE_1 Not English
  header   __BAD_LANGUAGE_1 X-Languages !~ /\b(en)\b/
  meta       BAD_LANGUAGE   (__HAS_LANGUAGE_1 && __BAD_LANGUAGE_1)
  score      BAD_LANGUAGE   4
  describe   BAD_LANGUAGE   Not English
else
  add_header all Language Plugin Not loaded
endif


# Custom regexps. I wish there were more concise ways than this.
# Rules beginning with __ are hidden from X-Spam-Report.
# Sub-rules of meta rules can't have score exactly 0 or they don't match.
#
score    __SUBJ_SPAM_1   0.01
meta       SUBJ_SPAM     (__SUBJ_SPAM_1)
score      SUBJ_SPAM     5
describe   SUBJ_SPAM     Spammy subject
describe __SUBJ_SPAM_1   Spammy subject

score    __BODY_SPAM_1   0.01
meta       BODY_SPAM     (__BODY_SPAM_1)
score      BODY_SPAM     5
describe   BODY_SPAM     Spammy body
describe __BODY_SPAM_1   Spammy body

score    __UNSUB_HEADER  0.01
score    __BODY_UNSUB_1  0.01
score    __BODY_UNSUB_2  0.01
meta       UNSUB         (__UNSUB_HEADER || __BODY_UNSUB_1 || __BODY_UNSUB_2)
score      UNSUB         4
describe   UNSUB         Message has unsubscribe stuff
describe __UNSUB_HEADER  List-Unsubscribe header present
describe __BODY_UNSUB_1  HTML with unsubscribe link
describe __BODY_UNSUB_2  Body has unsubscribe text

rawbody __BODY_UNSUB_1   />\s*Unsub/i

header  __SUBJ_SPAM_1    Subject =~ /Google Reviews|Quick Question|OneDrive/i

header  __UNSUB_HEADER   List-Unsubscribe =~ /./
body    __BODY_UNSUB_2   /Reply with STOP|receive any.?more email|please reply to this email|Click here to un|from future mailing|receive these email|you may unsub/

body   __BODY_SPAM_1     /optimize your business|attract more customers|best phone number|Dear Valued|brief survey|pre-paid card/i

# ...etc...

# Default for this is -6, which is probably too low.
score USER_IN_WELCOMELIST_TO -20
It seemed wrong to me that SpamAssassin was detecting DMARC_REJECT but Postfix wasn’t just rejecting those outright, so I briefly tried installing SPF-Engine and I can now recommend that you do that only if your intent is that you are never able to send or receive email again.
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Reference:

Apache SpamAssassin
Postfix