One of the most effective anti-spam measures one can implement is to enforce valid use of SMTP and other standards. Spam clients are interested in sending messages as quickly as possible, and so usually don't bother with actually implementing standards correctly. In this post I shall describe the various checks which can be used, show how to implement these checks in Postfix, and describe how to ensure that your mail server passes these checks when sending mail.

Reverse DNS Entries

RFC 1912 states that "every Internet-reachable host should have a name" and "make sure your PTR and A records match". This can be checked by performing a Forward Confirmed reverse DNS lookup¹. This check can be done before even accepting the TCP connection, which means the mail server's existence isn't even revealed to rejected clients.

Postfix: Add reject_unknown_client_hostname to smtpd_client_restrictions.
Passing: Ensure that your mail server has matching PTR and A records.

HELO/EHLO Hostname

RFC 2821 states that "a client SHOULD start an SMTP session by issuing the EHLO command". Almost all SMTP client implementations do do this, and so we can require the use of HELO/EHLO.

Postfix: Set smtpd_helo_required = yes.

RFC 2821 states that "the argument field [of the HELO/EHLO command] contains the fully-qualified domain name of the SMTP client if one is available". Since external mail servers have to be Internet reachable this is a requirement, and can be checked by looking up the name in DNS².

Postfix: Add reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname and reject_unknown_helo_hostname to smtpd_helo_restrictions.
Passing: Ensure that your mail server is configured to send a fully qualified hostname which exists in DNS.

If there is only one mail server (and possibly even if there are multiple servers), SMTP clients should not be using the server's hostname as the HELO hostname. Clients which do so can therefore be rejected.

Postfix: Add check_helo_access hash:/etc/postfix/helo_access to smtpd_helo_restrictions. Use helo_access as a template for /etc/postfix/helo_access, and run postmap /etc/postfix/helo_access afterwards.

Originator Address

The originator (MAIL FROM) address is where error reports will be sent, and therefore should be a valid address. The only thing which can be checked though is that the address is fully qualified and that the domain exists.

Postfix: Add reject_non_fqdn_sender and reject_unknown_sender_domain to smtpd_sender_restrictions.
Passing: Ensure that your mail server only emits fully qualified addresses. This should happen by default, except possibly for mail submitted with sendmail.

Recipient Addresses

Unless the mail server is a relay or backup MX, it should already only be accepting addresses for which it is the destination. If it is a relay or backup MX the same checks as above can be done.

Postfix: Add reject_non_fqdn_recipient and reject_unknown_recipient_domain to smtpd_recipient_restrictions.

One other check has to do with multiple recipients for bounced mail. Error reports for bounced mail uses a null originator address, and should only have one recipient.

Postfix: Add reject_multi_recipient_bounce to smtpd_data_restrictions.

Pipelining

Unless the client explicitly requests pipelining (as described in RFC 1854), the SMTP conversation must occur in lock step (i.e. the client must wait for a response from the server before sending the next command). Since spam clients are trying to send messages as quickly as possible it is likely that they do not adhere to this requirement.

Postfix: Add reject_unauth_pipelining to smtpd_data_restrictions.

RFC 2821 specifies that the server must send the first message after the connection is established. A neat trick is to delay this initial message to catch out clients which don't wait for it.

Postfix: Add sleep 1, reject_unauth_pipelining to smtpd_client_restrictions. This also requires smtpd_delay_reject = no (explained below).

Monitoring

Since these measures will reject valid mail from misconfigured mail servers, I like to keep an eye on rejections via logcheck. However, some of these measures by their very nature reject the client before it's even sent the originator and recipient addresses, which makes identification of valid mail difficult. Postfix therefore has a feature which delays rejection until after the recipient addresses have been sent. This is enabled by default, but can be disabled by setting smtpd_delay_reject = no.