Enabling Mail Submission from Remote Networks
To enable this, you'll want to add the remote networks to the zimbraMtaMyNetworks attribute on the server.
If this is not set, the postfix default is used.
Get the default:
su - zimbra postconf -d mynetworks
Check the existing value:
zmprov gs SERVERNAME | grep zimbraMtaMyNetworks
Add your new network or IP
zmprov ms SERVERNAME zimbraMtaMyNetworks "18.104.22.168/24 127.0.0.0/8 10.0.0.0/8"
NOTE - make sure that the 127.0.0.0/8 network is included. Quotes are important in the above command