By default, postfix is not configured for internal relaying with a binary Fedora Core 4 of Zimbra as far as I'm aware.
Is this to be manually configured or should it already come with an MTA that's capable of sending/receiving messages over the local interface?
Not sure what you mean? Do you want to relay mail with out DNS? IF so you can disable DNS and use a local MTA relay host.
Postfix's default configuration is not relaying any e-mail is what I merely meant. Should it work off of the bat after a binary installation or does the configuration need manual administrator intervention?
It works for me(and many others here). If you have DNS configured and use SMTP auth it works perfect. Maybe you'd like to post some error logs so we can try to troubleshoot your particular case.