Here are the configuration settings if you use Postfix as the MTA.
In
/etc/postfix/main.cf on server#1, add the following to relay mail to
server #2:relayhost=[ipv6_address_of_server2]:10025 default_destination_recipient_limit=100 default_destination_concurrency_limit=50
In
/opt/trend/imss/config/imss.ini, open connection restrictions and
point the downstream server IP to server #3:[socket] proxy_smtp_server_ip=all [smtp] smtp_allow_client_ip=127.0.0.1, ipv6_address_of_server1 downstream_smtp_server_addr=ipv6_address_of_server3
In
/etc/postfix/master.cf on server #3, modify smtpd settings to receive
mail on port 10026:10026 inet n - n - - smtpd