Running a mail server: Postfix itself, then the four things bolted on to it — two to keep other people's spam out, two to prove that outgoing mail really is yours.

• Postfix — a walk through main.cf section by section: directories, protocols and addresses, resources, queue ownership, host identity and home network, address rewriting, relaying, virtual domains and transport, SASL authentication, TLS, and the reception rules; then smtpd.conf and access.cf.
• Grey listing — SQLgrey: reject an unknown sender's first message and let a real sender try again later. Database initialisation, whitelist, startup, and the Postfix restriction that calls it.
• SpamAssassin — classifying mail by message analysis, blacklist queries and SPF records, and both ways of wiring it to Postfix: as a content filter and as a milter.
• Sender Policy Framework — publishing in DNS which hosts may send mail for the domain, and checking other people's records from Postfix.
• DomainKeys Identified Mail — signing outgoing mail with OpenDKIM so a recipient can tell it was neither forged nor modified: generating the signing key, publishing it, and the Postfix milter.
• Cyrus IMAP — the mailbox server itself: ZFS layout, the process table in cyrus.conf, TLS, virtual domains, sieve, quotas and the SNMP wiring. Partly in French, and its build options are still a TODO.