====== Setup Postfix to function as relay host ======
Setup Postfix to function as relay host with authentication
  * Create a password maps file
  * You can create multiple entries in the sasl_passwd file for multiple domains
<code>
echo "mailserver.example.com username:password" >> /etc/postfix/sasl_passwd
chown root:root /etc/postfix/sasl_passwd
chmod 600 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd
</code>

  * Update /etc/postfix/main.cf
<code>
relayhost = mailserver.example.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =
</code>

  * Restart/Reload postfix

===== Another Example =====
Typical configuration for an internal server (inside a LAN) that uses an SMTP server (also internal) to send mail.

Append this to main.cf
<code>
mydomain = mail.dev08.example.org
myhostname = mail.dev08.example.org
myorigin = $mydomain
append_dot_mydomain = no
relayhost = td006
smtp_host_lookup = native
</code>

Below is a configuration diff that was generated from a Ubuntu Trusty default Postfix install as "satellite" server. The server name is ubv10 (as see in diff below).  Replace server name throughout the main.cf file.   The local server relay is "localsmtp".  Change this as required.  Since this is a local server and the name does not feature in the DNS include the ''smtp_host_lookup'' option.  If you have a server name that resolves via DNS (as opposed to /etc/hosts), this is not required.  Of course make sure /etc/hosts has entry for localsmtp.

<code diff>
diff --git a/postfix/main.cf b/postfix/main.cf
index eb619b9..1539d7d 100644
--- a/postfix/main.cf
+++ b/postfix/main.cf
@@ -33,8 +33,12 @@ alias_maps = hash:/etc/aliases
 alias_database = hash:/etc/aliases
 myorigin = /etc/mailname
 mydestination = ubv10.localdomain, ubv10, localhost.localdomain, localhost
-relayhost = smtp.localdomain
+relayhost = localsmtp
+smtp_host_lookup = native
 mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
 mailbox_size_limit = 0
 recipient_delimiter = +
 inet_interfaces = loopback-only
</code>

Not exactly sure why, but sometimes it is required to include ''disable_dns_lookups = yes'' along with (or may instead of) ''smtp_host_lookup = native'' when using local (/etc/hosts) host resolution as opposed to DNS (MX record) resolution.
===== Generic mapping for outgoing SMTP mail =====
Since the from address will not appear as host.localdomain and be treated as junk mail, to rewrite relayed mail from address use ''smtp_generic_maps''.  Check the below reference.

Reference: http://www.postfix.org/ADDRESS_REWRITING_README.html#generic

Append to main.cf:
<code>
# Address rewriting
smtp_generic_maps = hash:/etc/postfix/generic
</code>

Once updated do hash and reload (as root):
<code bash>
postmap /etc/postfix/generic
/etc/init.d/postfix reload
</code>