On Sun, Oct 19, 2025 at 02:38:44PM +0200, Christian H. Kuhn via Postfix-users 
wrote:

> That fourth instance should listen on a unix socket, the three other
> instances should have a corresponding transport.

Unfortunately, that won't work.  The Postfix SMTP client does not
support unix-domain sockets in its nexthop syntax.  Only the LMTP client
supports that.  And IIRC the SMTP server may not support listening on
a unix-domain socket either.  It rather expects to be able to find and
and log an IP address and hostname for its client.

> default_transport = lmtp:unix:/var/spool/postfix-out/public/smtpd

But, the SMTP server does not speak LMTP.

> 2025-10-19T14:27:02.773551+02:00 bywater postfix/lmtp[2064542]: A13661E12A4:
> to=<[email protected]>, relay=none, delay=0.12,
> delays=0.11/0.01/0/0, dsn=4.4.1, status=deferred (connect to
> bywater.qno.de[/var/spool/postfix-out/public/smtpd]: No such file or
> directory)

You'd need an LMTP server for that, and Postfix does not presently have
one.

> As postfix and postfix-out are in different chroot, it is not possible to
> create a path from one chroot to the other, so i made a symbolic link:

Symlinks don't work across chroot jails.  Even with all the other
barriers resolved you'd also need to disable chroot.

> But there must be a way to transport mail via unix socket from one postfix
> instance to another?

There isn't unless you implement an LMTP server, or teach the Postfix
SMTP client and server to support unix-domain sockets.

-- 
    Viktor.  🇺🇦 Слава Україні!
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