Relay per Domain
Reference bug: http://bugzilla.zimbra.com/show_bug.cgi?id=32740
Zimbra allows relaying emails to a specific server only, i.e using zimbraMtaRelayHost. However, there are needs to relay emails per domain basis based on *the sender*. For example, you want emails sent by firstname.lastname@example.org should be relayed through smtp.domain1.com and email@example.com through smtp.domain2.com servers. You can configure it using postfix's "sender_dependent_relayhost_maps".
This is an example and tested on ZCS 5.0.16
1. Add following line to /opt/zimbra/postfix/conf/main.cf
sender_dependent_relayhost_maps = hash:/opt/zimbra/postfix/conf/bysender
2. Create file /opt/zimbra/postfix/conf/bysender and enter your domain names and relay server's IP addresses.
@domain1.com [10.10.10.1] @domain2.com [18.104.22.168]
You can even add individual email IDs to relay their emails to specific relay host.
3. Create the hash file.
4. Restart zmmtactl
zmmtactl stop zmmtactl start
5. Test by sending emails.
Note: Above settings will not survive zimbra upgrades. Make sure you take backup of config files before upgrading.