- http://planet.debian.org/
- http://www.debuntu.org/postfix-and-postgrey-a-proactive-approach-to-spam-filtering
- http://www.postfix.org/SMTPD_POLICY_README.html#greylist
- http://www.rexxinfo.org/
- http://www.markcrocker.com/rexxtipsntricks/rxtt28.1.toc.html
- SPF Setup Wizard
- Automating backups with tar
- http://www.openldap.org/doc/admin23/security.html
- https://help.ubuntu.com/community/OpenLDAPServer