Thanks a lot for the help. There is no firewall messing with SMTP inbetween. With both options

resolve_dequoted_address = no
allow_untrusted_routing = yes

it finally works. Because I don't have a backup MX, this set-up should be fairly safe to use.


Am 03.01.2013 22:08, schrieb Wietse Venema:
Michael Blessenohl:
I'm sorry, I'll try not to use my smartphone again to answer mails from
this list.

I ment using a remote machine as client to connect to the postfix server
as opposed to connect to the machine itself. I don't know why it does
matter, but apparently it does. Using the hostname, IP or localhost
makes no difference at all. That's the same for me.
You may want to look into the following parameter.

allow_untrusted_routing (default: no)
    Forward  mail  with sender-specified routing (user[@%!]remote[@%!]site)
    from untrusted clients to destinations matching $relay_domains.

    By default, this feature is turned off.  This closes a nasty
    open relay loophole  where  a  backup  MX host can be tricked
    into forwarding junk mail to a primary MX host which then spams
    it out to the world.

Postfix flags an address with @ in the local-part as an address
with sender-specified routing, regardless of whether it is quoted.

Postfix will not relay such an address unless the above safety
feature is turned off.

        Wietse

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