I know there is a flag on SMTP that leaves you send mail trough a server using Telnet login on port 25.
On my zimbra server i cannot send to a foreign address (because relay in blocked), but i can send to an internal (existing) mail address (and if this is a distribution list...then i can send to foreign addresses...)

For example:
-------------------------------------
telnet mail.myserver.com 25
Trying x.x.x.x
Connected to mail.myserver.com.
Escape character is '^]'.
220 mail.myserver.com ESMTP Postfix
HELO abc
250 mail.myserver.com
MAIL FROM:
250 2.1.0 Ok
RCPT TO:
250 2.1.5 Ok
DATA
354 End data with .
Subject: Test
Hello.
.
250 2.0.0 Ok: queued as A8999999999
QUIT
221 2.0.0 Bye
Connection closed by foreign host.
-------------------------------------

Doing this (from anywhere you want... local lan, outside...), the user ANYEXISTINGADDRESS@myserver.com receives an internal mail from anyaddress@myserver.com...

As i said, I know tha this is the way smtp works (if the RCPT TO is inside the local server, the mail will be delivered). But, is there any way to lock/disable this for outside connections?