I recently installed Zimbra Open source Edition 8.0.2 on Ubuntu 12.04.

We suppose the fowolling on my zimbra server
Domain: test.com
Users: user1@test.com, distribution-list@test.com

From an external network I open telnet on my zimbra server with port 25.
HELO ....
250 mail.test.com
MAIL FROM: user1@test.com
250 2.1.0 Ok
RCPT TO: distribution@test.com
250 2.1.0 Ok

I wan't to force smtp auth in this case. If an external person knows about the existence of user1@test.com he can send spam to my distribution-list

These parameters are set:
zimbraMtaAuthEnabled: TRUE
zimbraMtaAuthTarget: TRUE
zimbraMtaSaslAuthEnable: yes
zimbraMtaTlsAuthOnly: TRUE
zimbraShareNotificationMtaAuthRequired: FALSE

Thanks for helping,