2 - Raspberry PI Community Projects

11.1.3.3. Accepting or Refusing Based on the Announced Sender
Every message has a sender, announced by the MAIL FROM command of the SMTP protocol;
again, this information can be validated in several different ways.
smtpd_sender_restrictions =
check_sender_access hash:/etc/postfix/access_sender,
reject_unknown_sender_domain, reject_unlisted_sender,
reject_non_fqdn_sender
Example 11.8
Sender checks
The /etc/postfix/access_sender table maps some special treatment to some senders. This
usually means listing some senders into a white list or a black list.
The reject_unknown_sender_domain rule requires a valid sender domain, since it is needed
for a valid address. The reject_unlisted_sender rule rejects local senders if the address does not
exist; this prevents emails from being sent from an invalid address in the falcot.com domain,
and messages emanating from joe.bloggs@falcot.com are only accepted if such an address really
exists.
Finally, the reject_non_fqdn_sender rule rejects emails purporting to come from addresses
without a fully-qualified domain name. In practice, this means rejecting emails coming from
user@machine: the address must be announced as either user@machine.example.com or
user@example.com.
11.1.3.4. Accepting or Refusing Based on the Recipient
Each email has at least one recipient, announced with the RCPT TO command in the SMTP
protocol. These addresses also warrant validation, even if that may be less relevant than the
checks made on the sender address.
smtpd_recipient_restrictions = permit_mynetworks,
reject_unauth_destination, reject_unlisted_recipient,
reject_non_fqdn_recipient
Example 11.9
Recipient checks
reject_unauth_destination is the basic rule that requires outside messages to be addressed to
us; messages sent to an address not served by this server are rejected. Without this rule, a server
becomes an open relay that allows spammers to sent unsolicited emails; this rule is therefore
strongly recommended, and it will be located near the beginning of the list for preference, so
as to avoid other rules to authorize the message to pass through before its destination has been
checked.
260 The Debian Administrator's Handbook