From: Wietse Venema (wietseporcupine.org)
Date: Thu Sep 20 2007 - 10:44:42 CDT

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Bill Landry:
> Mark Martinec wrote:
> >>> amavisd unix - - n - 5 smtp
> >>> -o disable_mime_output_conversion=yes
> >>> ...
> >> After adding "smtpd_discard_ehlo_keywords = ('8BITMIME');" to
> >> amavisd-new, I realized that I needed to change the last line above to
> >> "disable_mime_output_conversion=no" and that seems to have resolved the
> >> signing issue I was having with 8bit mail sent from SquirrelMail.
> >
> > Right. This option was never suggested for a signing path
> > in amavisd-new documentation, quite the opposite.
>
> When I set up the DK and DKIM milters, I followed the advise shown in the
> Postfix MILTER_README, which states:
>
> "Content filters may break domain key etc. signatures. If you use an SMTP-based
> content filter, then you should add a line to master.cf with "-
> o disable_mime_output_conversion=yes" (note: no spaces around the "="), as
> described in the advanced content filter example."

This manual is about 8->7 conversion AFTER signing.

disable_mime_output_conversion=yes will prevent Postfix from breaking
a EXISTING signatures when it sends mail into a filter that is
8-bit clean, but that doesn't announce 8BITMIME support.

Mark is concerned with 8->7 conversion BEFORE signing.

In that case, converting 8bit to 7bit may be preferable, because
mail is less likely do be beoken due to mime conversion elsewhere.
This is safe if you are absolutely sure that you're not handling
mail that already has some digial signature (PGP clear sign, etc.).

        Wietse

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