Protecting internal email distribution lists [Postfix]

We want to implement an internal email distribution list. Something like all@our.domain.com, which aliases to all employees. My first thought was to use the aliases map, but that would lead to "all" being accessible from the "outside", and this is not desired... :-)


Option:01

Postfix can implement per-address access controls. What follows is based on the SMTP client IP address, and therefore is subject to IP spoofing.

/etc/postfix/main.cf:
    smtpd_recipient_restrictions =
        ...
        check_recipient_access hash:/etc/postfix/access
        ...the usual stuff...

/etc/postfix/access:
    all@example.com   permit_mynetworks,reject

# postmap /etc/postfix/access

Remarks: This rule only permits my_networks user to send mail to  all@example.com email address. All outsiders will be rejected.


Option:02

In the general case you need two lookup tables: one table that lists destinations that need to be protected, and one table that lists domains that are allowed to send to the protected destinations.

What follows is based on the sender SMTP envelope address, and therefore is subject to SMTP sender spoofing.

/etc/postfix/main.cf:
    smtpd_recipient_restrictions =
        ...
        check_recipient_access hash:/etc/postfix/protected_destinations
        ...the usual stuff...

    smtpd_restriction_classes = insiders_only
    insiders_only = check_sender_access hash:/etc/postfix/insiders, reject

/etc/postfix/protected_destinations:
    all@my.domain   insiders_only
    all@my.hostname insiders_only

/etc/postfix/insiders:
    my.domain       OK  matches my.domain and subdomains

    another.domain  OK  matches another.domain and subdomains


Ref:
http://www.postfix.org/RESTRICTION_CLASS_README.html