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Per default, mailcow considers all private RFC1918 networks (i.e. 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) as trusted. Though it is reasonable in most cases, you may want to restrict this setting under certain circumstances. In particular, if you are using some kind of reverse proxy for SMTP TCP ports. If your reverse proxy host is located in a private net, mailcow will consider all traffic from it as trusted, which may result in an open relay.
To change this behaviour override the default value of mynetworks parameter through the data/conf/postfix/extra.cf configuration file.
Important: Do not remove the networks listed as IPV4_NETWORK and IPV6_NETWORK in your mailcow.conf. You should also keep local addresses.
The default values for those variables - 172.22.1.0/24 and fd4d:6169:6c63:6f77::/64 - would result in the following, minimal configuration:
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 [fe80::]/10 172.22.1.0/24 [fd4d:6169:6c63:6f77::]/64