On Sun, Jan 30, 2022 at 02:39:32PM +0100, Christoph Pleger wrote:
> the advanced content filter documentation in
> http://www.postfix.org/FILTER_README.html describes how an email is
> passed to a content filter listening on a TCP port on localhost, but
> how is the email passed if the content filter is listening on a unix
> socket?
The Postfix smtp(8) client is TCP-only. Delivery to unix-domain sockets
is supported by lmtp(8), so your content-filter would need to support
the closely-related "LMTP" protocol.
> content_filter = proxsmtp:unix:external/proxsmtpd.sock
This is expected to fail.
> The other way round, passing from the filter to postfix only using UNIX
> sockets, is working.
You're mistaken. On input, Postfix provides no LMTP server, and no
support for receiving messages from external programs via unix-domain
sockets. Inter-process communication *internal* to Postfix (e.g.
between the queue manager and delivery agents) uses unix-domain sockets,
but these are not available as outward-facing services.
Messages can only come in via:
1. TCP via smtpd(8)
2. TCP via qmqpd(8)
3. sendmail(1) reading from a file or pipe
--
Viktor.
