Viktor Dukhovni via Postfix-users:
> On Tue, Jul 18, 2023 at 01:43:46PM +0200, Tinne11 via Postfix-users wrote:
>
> > In order to follow this recommendation, a Postfix MSA (being part of a
> > system DKIM-signing outbound messages) needs to be configured to convert all
> > submitted 8-bit messages to 7-bit (base64 or Quoted-Printable). Is this
> > possible?
>
> Yes, but it requires a (possibly NOOP) content_filter delivery hop,
> or equivalently a multi-instance configuratoin with an input and output
> Postfix instance. If you force all incoming mail through an internal
> SMTP hop, you can do the 8bit -> 7bit downgrade during that delivery,
> and delay the DKIM signing until the output instance, which will
> always see 7bit input.
The downgrade can be forced by sending mail from Postfix using an
SMTP client that ignores the server's 8BITMIME announcement (with
"smtp_discard_ehlo_keywords = 8BITMIME").
This can work with the 'advanced' example in FILTER_README:
main.cf:
content_filter = smtp-7bit:127.0.0.1:10025
master.cf:
smtp-7bit .. .. .. .. .. .. smtp
-o { smtp_discard_ehlo_keywords = 8BITMIME }
127.0.0.1:10025 .. .. .. .. .. .. smtpd
-o { content_filter= }
And with multiple Postfix instances.
Wietse
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