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autoconvert messes up DKIM signature

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Hello sendmail experts!

Today I sent myself a test email from an updated k9-mail client and imagine my 
surprise as the DKIM check failed with "Invalid (E-mail was modified)".

I checked the mail headers and found the following:

X-MIME-Autoconverted: from quoted-printable to 8bit by [bla] 59VKaHmW832917

Weird, why would sendmail do such a conversion? I never told it to do so (or at 
least I think I never did). I guess k9-mail sent the mail in a format sendmail 
didn't like. However, there is nothing I can do about that. But what is even 
weirder is that sendmail modifies a signed email. So this basically breaks DKIM.

This brought a few interesting questions to light, some of which most likely 
show my non-sendmail-expert status. So please bear with me:

1. Why would sendmail modify the mail after it was signed? Is there a way that 
sendmail does this conversion before the opendkim milter signs it? I can't be 
the only one who uses opendkim and sendmail. Modifying an email after is was 
signed clearly makes no sense.

2. Is there a reason for sendmail to do this conversion in the first place? 
Afaik quoted-printable is not an exotic/unknown format and thus should be 
understood by today's clients.

3. How can I turn off this auto-conversion? Would it result in drawbacks?

While the 3rd one is the brutal approach, I'd rather go with the 1st one, which 
also makes the most sense in terms of a mail processing workflow.
Unfortunately I don't know how to do either.

Here is my sendmail.mc: https://evermeet.cx/paste/sendmail_mc.xD5s.txt 
(available until 2025-11-07 20:56:45 UTC)

Cheers,
   K. C.

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autoconvert messes up DKIM signature "Helmut K. C. Tessarek" <kct@evermeet.cx> - 2025-10-31 17:21 -0400
  Re: autoconvert messes up DKIM signature "Helmut K. C. Tessarek" <kct@evermeet.cx> - 2025-10-31 19:40 -0400
    Re: autoconvert messes up DKIM signature Marco Moock <mm+solani@dorfdsl.de> - 2025-11-01 10:39 +0100
      Re: autoconvert messes up DKIM signature "Helmut K. C. Tessarek" <kct@evermeet.cx> - 2025-11-01 22:03 -0400
        Re: autoconvert messes up DKIM signature Claus Aßmann <INVALID_NO_CC_REMOVE_IF_YOU_DO_NOT_POST_ml+sendmail(-no-copies-please)@esmtp.org> - 2025-11-02 01:38 -0400

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