Zimbra Version : Release 7.1.0_GA_3140.RHEL5_64_20110329150833 CentOS5_64 FOSS edition.

Edit postfix_transport_maps values :

[root@mail conf]# su zimbra
[zimbra@mail conf]$ zmlocalconfig -e postfix_transport_maps=hash:\${zimbra_home}/conf/transport,proxy:ldap:\${zimbra_home}/conf/ldap-transport.cf

Create file transport in /opt/zimbra/conf

[zimbra@mail conf]$ vi /opt/zimbra/conf/transport

Add this line to file transport (Format -> destinationdomain :[smtprelayhost]) sample :

gmx.com :[smtp.server.net.id]

Convert the transport file into maptype database file :

[zimbra@mail conf]$ postmap /opt/zimbra/conf/transport

Restart MTA

[zimbra@mail conf]$ zmmtactl restart
Rewriting configuration files...done.
postfix/postfix-script: refreshing the Postfix mail system
Stopping saslauthd...done.
Starting saslauthd...done.
[zimbra@mail conf]$

Source : http://wiki.zimbra.com/index.php?title=Transport_Table_for_external_servers