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Mail transfer agents (MTAs) are servers that are used to receive local e-mail and send outgoing e-mail.
myhostname = The hostname of the mail server.
Default: If a fully qualified domain name (FQDN) is configured on the Unix machine then Postfix will use that.
mynetworks = Define the networks that are allowed to send e-mails.
mydestination = Define allowed destinations for e-mails.
$myhostname localhost.$mydomain localhost
relayhost = Define the server to forward mail to. 
In a null client configuration, mail is not accepted by the mail server. It only forwards local e-mails to another MTA.
Example - Forward all e-mails from the localhost to the server at the 10.0.0.1 address.
myhostname = server.rootpages.tld mydomain = rootpages.tld myorigin = $mydomain inet_interfaces = loopback-only mydestination = relayhost = 10.0.0.1
In a gateway-client setup, mail is forwarded from the one or more specified networks to another MTA.
Example - Forward all e-mails from localhost and from the 10.0.0.0/24 network to the 10.0.0.1 server.
myhostname = server.example.com mydomain = example.com myorigin = $mydomain inet_interfaces = all mydestination = $myhostname localhost.$mydomain localhost $mydomain mynetworks = 10.0.0.0/24 127.0.0.0/8 relayhost = 10.0.0.1
“Postfix Basic Configuration.” Postfix. Accessed October 1, 2016. http://www.postfix.org/BASIC_CONFIGURATION_README.html
“Postfix Standard Configuration.” Postfix. Accessed October 1, 2016. http://www.postfix.org/STANDARD_CONFIGURATION_README.html
“Configure a system to forward all email to a central mail server.” CertDepot. November 25, 2015. Accessed October 1, 2016. https://www.certdepot.net/rhel7-configure-system-forward-email-central-mail-server/