default isses so genau richtig, wenn Du genau diese Funktion brauchst->
Postfix Configuration Parameters
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relayhost (default: empty)
The next-hop destination of non-local mail; overrides non-local domains in recipient addresses. This information is overruled with relay_transport, sender_dependent_default_transport_maps, default_transport, sender_dependent_relayhost_maps and with the transport(5) table.
On an intranet, specify the organizational domain name. If your internal DNS uses no MX records, specify the name of the intranet gateway host instead.
In the case of SMTP, specify a domain name, hostname, hostname
ort, [hostname]
ort, [hostaddress] or [hostaddress]
ort. The form [hostname] turns off MX lookups.
If you're connected via UUCP, see the UUCP_README file for useful information.
Examples:
relayhost = $mydomain
relayhost = [gateway.example.com]
relayhost = uucphost
relayhost = [an.ip.add.ress]
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Gruß Sven