2 - Raspberry PI Community Projects

The reject_unlisted_recipient rule rejects messages sent to non-existing local users, which
makes sense. Finally, the reject_non_fqdn_recipient rule rejects non-fully-qualified addresses;
this makes it impossible to send an email to jean or jean@machine, and requires using the full
address instead, such as jean@machine.falcot.com or jean@falcot.com.
11.1.3.5. Restrictions Associated with the DATA Command
The DATA command of SMTP is emitted before the contents of the message. It doesn't provide
any information per se, apart from announcing what comes next. It can still be subjected to
checks.
smtpd_data_restrictions = reject_unauth_pipelining
Example 11.10
DATA checks
The reject_unauth_pipelining directives causes the message to be rejected if the sending party
sends a command before the reply to the previous command has been sent. This guards against a
common optimization used by spammer robots, since they usually don't care a fig about replies
and only focus on sending as many emails as possible in as short a time as possible.
11.1.3.6. Applying Restrictions
Although the above commands validate information at various stages of the SMTP exchange,
Postfix only sends the actual rejection as a reply to the RCPT TO command.
This means that even if the message is rejected due to an invalid EHLO command, Postfix knows
the sender and the recipient when announcing the rejection. It can then log a more explicit
message than it could if the transaction had been interrupted from the start. In addition, a
number of SMTP clients do not expect failures on the early SMTP commands, and these clients
will be less disturbed by this late rejection.
A final advantage to this choice is that the rules can accumulate information during the various
stages of SMTP; this allows defining more fine-grained permissions, such as rejecting a non-local
connection if it announces itself with a local sender.
11.1.3.7. Filtering Based on the Message Contents
QUICK LOOK
Regexp tables
The /usr/share/doc/postfix-doc/examples/header_checks.gz file contains
many explanatory comments and can be used as a starting point for creating
the /etc/postfix/header_checks and /etc/postfix/body_checks files.
The validation and restriction system would not be complete without a way to apply checks
to the message contents. Postfix differentiates the checks applying on the email headers from
Chapter 11 — Network Services: Postfix, Apache, NFS, Samba, Squid, LDAP
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