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SysAdmin Blog

sendmail MIME conversion vs. DMARC+DKIM

Alexander Bochmann Saturday 15 of April, 2017
I've recently tried to reconfigure a mailinglist that I run on one of my systems to make less problems with recipients that use DMARC.

To that end, I wanted to implement the first option mentioned in the corresponding DMARC FAQ for mailinglist administrators (cache): Don't do any changes to the message body and headers potentially covered by a DKIM signature.

On the mailinglist configuration side, this means not adding a list tag to the message subject, and not changing the body with an additional header or footer. (Adding the usual RFC2369/RFC2919 list headers is no problem.)

After doing that it turned out that my setup, using sendmail, still changed the body on some messages, thanks to the automatic MIME autoconversion that sendmail does.

Getting rid of that actually required some digging into sendmail configuration and documentation:

Responsible for message delivery to local programs is the aptly named "prog" mailer. Unfortunately, as far as the m4 configuration statements are concerned, all the variables for this mailer are called "SHELL"... The default flags for the prog/shell mailer are hardcoded to "[eu9]", according to the README (though these defaults are changed in some of the OSTYPE definitions in the m4 macro collection):

sendmail cf.README wrote:
LOCAL_SHELL_FLAGS [eu9] The flags used by the shell mailer. The flags lsDFM are always included.

The meaning of the individual flags is documented in the OP manual:

sendmail OP manual wrote:
7
   Strip all output to seven bits. This is the default if the L flag is set. Note that clearing this option is not sufficient to get full eight bit data passed through sendmail. If the 7 option is set, this is essentially always set, since the eighth bit was stripped on input. Note that this option will only impact messages that didn't have 8->7 bit MIME conversions performed.
8
   If set, it is acceptable to send eight bit data to this mailer; the usual attempt to do 8->7 bit MIME conversions will be bypassed.
9
   If set, do limited 7->8 bit MIME conversions. These conversions are limited to text/plain data.


So it seems I want to get rid of the 9 in the LOCAL_SHELL_FLAGS, and replace it by a 8...

Adding the following two statements to my sendmail m4 configuration source does exactly that:

sendmail.mc
dnl # disable MIME-Autoconversion for prog mailer
MODIFY_MAILER_FLAGS(`SHELL', `-9')
MODIFY_MAILER_FLAGS(`SHELL', `+8')