I'm trying to prevent anonymous users from potentially being able to send email from one local user to another. If someone knows the email addresses of 2 accounts on my postfix server (2.10.1) it seems that they are able to send email between them from anywhere without authenticating. I tested this using telnet commands:
220 domain1.co.uk ESMTP Postfix
250-AUTH PLAIN LOGIN
250 2.1.0 Ok
250 2.1.5 Ok
354 End data with .
250 2.0.0 Ok: queued as A002982019
221 2.0.0 Bye
I want to avoid anyone from being able to send between local domains unless they are either authenticated or from specific IP addresses (local ones for relay).