Aquí muestro las webs más importantes que he consultado. Puede que en el futuro algunas ya no estén disponibles, pero merece la pena mencionarlas:

Configuring postfix – tldp.com : http://www.tldp.org/HOWTO/Postfix-Cyrus-Web-cyradm-HOWTO/postfix-config.html

Postfix/master.cf – HTYP: http://htyp.org/Postfix/master.cf

Postfix mail server limit the mailbox size : http://www.cyberciti.biz/tips/postfix-mail-server-limit-the-mailbox-size.html

Linux postfix SMTP : http://www.cyberciti.biz/tips/postfix-smtp-ssl-certificate-csr-installation-guide.html

Postfix configure anti spam with blacklist : http://www.cyberciti.biz/tips/postfix-spam-filtering-with-blacklists-howto.html

IntegratedSpamdInPostfix – SpamAssassin Wiki : http://wiki.apache.org/spamassassin/IntegratedSpamdInPostfix

Fighting Spam with SpamAssassin and Postfix : http://www.akadia.com/services/postfix_spamassassin.html

SpamAssassin – Using pyzor:  http://wiki.apache.org/spamassassin/UsingPyzor

SpamAssassin – Using SQL : http://wiki.apache.org/spamassassin/UsingSQL

Automatically purge old emails – Kolab Wiki : http://wiki.apache.org/spamassassin/UsingSQL

Sieve server-side mail filtering with Postfix, Dovetot – Koansys : http://koansys.com/tech/sieve-server-side-mail-filtering-with-postfix-dovecot

Sieve (mail filtering language) – Wikipedia : http://en.wikipedia.org/wiki/Sieve_%28mail_filtering_language%29

SpamAssassin README: http://cpansearch.perl.org/src/FELICITY/Mail-SpamAssassin-3.0.2/sql/README.bayes

Instalando un antispam con SpamAssassin y Postfix : http://www.imaginanet.com/blog/instalando-un-antispam-con-spamassassin-y-postfix.html