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What exactly do you want to happen? If the mail is NOT sent from a known local user, or from a known alias, then you want the mail dropped, or simply sent from the 'unknown' user (such as a Spam sender would want)?
You'll wanna look at the 'mynetworks' parameter, and specify the subnet (or class or other as noted in the main.cf) you want to allow mail from, this should be outlined in your main.cf but here's the exerpt to note:
# By default, Postfix relays mail
# - from "trusted" clients (IP address matches $mynetworks) to any destination,
# - from "untrusted" clients to destinations that match $relay_domains or
# subdomains thereof, except addresses with sender-specified routing.
# The default relay_domains value is $mydestination.
Which pertains to:
# TRUST AND RELAY CONTROL
# The mynetworks parameter specifies the list of "trusted" SMTP
# clients that have more privileges than "strangers".
# In particular, "trusted" SMTP clients are allowed to relay mail
# through Postfix. See the smtpd_recipient_restrictions parameter
# in file sample-smtpd.cf.
# You can specify the list of "trusted" network addresses by hand
# or you can let Postfix do it for you (which is the default).
# By default (mynetworks_style = subnet), Postfix "trusts" SMTP
# clients in the same IP subnetworks as the local machine.
# On Linux, this does works correctly only with interfaces specified
# with the "ifconfig" command.
# Specify "mynetworks_style = class" when Postfix should "trust" SMTP
# clients in the same IP class A/B/C networks as the local machine.
# Don't do this with a dialup site - it would cause Postfix to "trust"
# your entire provider's network. Instead, specify an explicit
# mynetworks list by hand, as described below.
# Specify "mynetworks_style = host" when Postfix should "trust"
# only the local machine.
#mynetworks_style = class
#mynetworks_style = subnet
#mynetworks_style = host
# Alternatively, you can specify the mynetworks list by hand, in
# which case Postfix ignores the mynetworks_style setting.
# Specify an explicit list of network/netmask patterns, where the
# mask specifies the number of bits in the network part of a host
If you've defined mynetworks it will override mynetworks_style. Either way, be sure to restart postfix after making changes to the main.cf:
After that you **should** be able to relay, the above error shouldn't exist on **outgoing** email.
Just read over the FAQ and documentation, and I still think this is true, however, can you post up your full /etc/postfix/main.cf so we can see what's setup? If you feel it necessary, go ahead and use "example.net" instead of your actual domain where all references exist.