Instead of using the /etc/postfix/transport flat file for routing, Zimbra allows you to set this via the command line:
./zmprov cd excite.com zimbraMailTransport smtp:xmxatip.excite.com zimbraDomainType transport
In this case here, the mail domain we’re changing the MTA to is xmxatip.excite.com and the domain is excite.com so that all email to excite.com will go to xmxatip.excite.com.
What changed on the back end is that there’s an entry written to the ldap server:
# excite.com
dn: dc=excite,dc=com
zimbraMailStatus: enabled
zimbraId: 50243e3e-2435-4366-8c18-33697b15f136
dc: excite
zimbraDomainName: excite.com
zimbraDomainType: transport
zimbraMailTransport: smtp:xmxatip.excite.com
objectClass: dcObject
objectClass: organization
objectClass: zimbraDomain
o: excite.com domain
# people, excite.com
dn: ou=people,dc=excite,dc=com
ou: people
objectClass: organizationalRole
cn: people