dkim=fail (body hash did not verify)

mailauth writes `dkim=fail (body hash did not verify)` in strict mode and `dkim=neutral (body hash did not verify)` in its default lenient mode (both reproduced with 8.0.1). OpenDKIM reports the same situation as `dkim=fail reason="signature verification failed"` (reproduced with 2.11.0).

DKIM verifiers (Gmail, mailauth, Rspamd…) — Authentication-Results after the body changed in transit

What it means

The `bh=` tag of the DKIM signature holds a hash of the message body as the sender signed it. The receiver hashed the body it received (after canonicalization) and got a different value, so the body changed in transit. The header signature was not even the problem: RFC 6376 makes a body-hash mismatch a permanent failure. Classic culprits are mailing-list footers, disclaimers and security gateways that rewrite links.

Why it happens

• A footer or disclaimer was appended

  common

  Mailing lists (unsubscribe footer), corporate disclaimers added by an outbound gateway, or "Sent from…" lines added by relays change the canonicalized body.

• A security gateway rewrote the body

  common

  Link rewriting (Safe Links, URL defense), attachment stripping or banner insertion by inbound or outbound filters.

• The MIME transfer encoding changed

  occasional

  A relay without 8BITMIME converts 8-bit content to quoted-printable or base64. The decoded text is identical; the signed bytes are not.

• The signer hashed different line endings

  occasional

  Signing code that hashes LF line endings while the message is sent with CRLF (or with trailing whitespace stripped later) produces a body hash that never matches with `simple` canonicalization.

How to fix it

1. 1.

   Recompute bh= yourself

   Comparing a recomputed body hash on the sent copy and on the received copy tells you whether the signer or a relay is at fault.

   # Recompute bh= for relaxed body canonicalization and compare with the header's bh= value
   python3 - message.eml <<'EOF'
   import base64, hashlib, re, sys
   raw = open(sys.argv[1], "rb").read().replace(b"\r\n", b"\n")
   body = raw.split(b"\n\n", 1)[1]
   lines = [re.sub(rb"[ \t]+", b" ", l).rstrip(b" \t") for l in body.split(b"\n")]
   canon = b"\r\n".join(lines).rstrip(b"\r\n") + b"\r\n"
   print("computed bh=", base64.b64encode(hashlib.sha256(canon).digest()).decode())
   EOF
   grep -o "bh=[^;]*" message.eml

2. 2.

   Sign after every modification

   Make the DKIM signer the last step on your outbound path; for intermediaries you do not control, rely on ARC and DMARC alignment via SPF where possible.

   # Postfix: run content filters (disclaimers, antivirus) BEFORE the DKIM signing milter
   smtpd_milters = inet:127.0.0.1:10025, inet:127.0.0.1:8891
   #               ^ disclaimer milter   ^ OpenDKIM (signs last)

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