We want to implement an internal email distribution list.
Something like [email protected], 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... :-)
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:
[email protected]
permit_mynetworks,reject
[email protected]
permit_mynetworks,reject
Specify dbm instead of hash if your system uses
dbm files instead of db files. To find out what map
types Postfix supports, use the command postconf -m.
Now, that would be sufficient when your machine receives all
Internet mail directly from the Internet. That's unlikely if your
network is a bit larger than an office. For example, your backup
MX hosts would "launder" the client IP address of mail from the
outside so it would appear to come from a trusted machine.
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:
[email protected] insiders_only
[email protected] insiders_only
/etc/postfix/insiders:
my.domain OK matches my.domain and subdomains
another.domain OK matches another.domain and subdomains
Getting past this scheme is relatively easy, because all one
has to do is to spoof the SMTP sender address.
If the internal list is a low-volume one, perhaps it makes more
sense to make it moderated.
