SMTP relay from cpanel and mailcow as smarthost
cpanel
https://blog.cpanel.com/how-to-set-up-a-smarthost-smtp-relay-with-cpanel/
mailcow
nano /opt/mailcow-dockerized/data/conf/postfix/extra.cf
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::]/64 192.168.2.0/24
https://mailcow.github.io/mailcow-dockerized-docs/u_e-postfix-trust_networks/