Send email through Gateway server.

1)Enable "SMTP used when sending message outside of the local internet domain" on Gateway server.
Configuration-Server-Configuration->select server->Router/SMTP->Basic
2)Enable "SMTP listener" on Gateway server
configuration->all server document->select server->basic
3)Set replay policy on Gateway server
Configuration-Server-Configuration->select server->Router/SMTP->Restrictions & Controls..->SMTP inbound controls->Allow messages only from the following internet hosts to be sent to external internet domain (input the ip address of mail server)
3)Enable "SMTP used when sending message outside of the local internet domain" on mail server
Set relay host (Gateway server's Ip address)
Configuration-Server-Configuration->select server->Router/SMTP->Basic