I have done a bunch more research to figure out what exactly is causing
ASSP to generate invalid DKIM signatures. I can't say for sure what is
causing it, but I can give an example of a very simple email which does
cause it consistently:
When this very simple email is transmitted as the body, the message
signing fails every time:
----=43D70E2DE97749F99004_B859_B29B_9ACB
Content-Type: text/html;charset="UTF-8"
Content-Transfer-Encoding: 7bit
<table>
<tr>
<td>
</td></tr>
</table>
----=43D70E2DE97749F99004_B859_B29B_9ACB--
It is being sent as multipart/mixed, and the content inside the
boundaries is the only part contained within this email. GMail reports this:
ARC-Authentication-Results: i=1; mx.google.com; dkim=neutral (body hash
did not verify)
HOWEVER...If I remove the blank line as shown below, the message
signature passes fine:
----=5B9E2600A819445B9009_1861_9C26_1181
Content-Type: text/html;charset="UTF-8"
Content-Transfer-Encoding: 7bit
<table>
<tr>
<td>
</td></tr>
</table>
----=5B9E2600A819445B9009_1861_9C26_1181--
The blank line shown consists of CR/LF/LF: 0d 0a 0a
If I change the 0d 0a 0a in the email to 0d 0a 0d 0a, the signature now
succeeds.
However, when I edit the consecutive LF's out of the newsletter email,
ASSP still generates an invalid DKIM signature when sending it, so that
is apparently not the only thing that is tripping up ASSP when
generating signatures.
On 5/1/2026 10:59, Scott MacLean wrote:
I send out a newsletter to over 100,000 people once a month. This month
has been an utter disaster.
For some reason, when ASSP generates the DKIM signature for this month's
newsletter, it generates it incorrectly. As a result, the newsletter has
been rejected by all major email systems (most notably Google and
Microsoft) - and as a result of the mass rejections, my server is now
blacklisted all over the place.
This is what Google is saying:
DKIM: 'FAIL'
Authentication-Results: mx.google.com; dkim=neutral (body hash did not
verify)
I can send other emails to Google through my server and they are signed
properly and pass. But when I send this specific email, containing this
month's newsletter, the DKIM signature created by ASSP is consistently,
repeatedly wrong, and is rejected everywhere.
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