Postfix

Postfix is a free, open-source mail transfer agent (MTA) that routes and delivers electronic mail, designed as a secure, fast, and easy-to-administer alternative to Sendmail. Created by Wietse Venema (a renowned security researcher at IBM Research and creator of TCP Wrapper and SATAN) in 1997, Postfix has become the most widely used MTA on the internet, powering email delivery for major ISPs, enterprises, and cloud providers. Postfix's architecture consists of multiple small, modular programs (master daemon managing child processes, smtpd for receiving, smtp for sending, qmgr for queue management, cleanup for header/body processing, local for local delivery, virtual for virtual domain delivery, and numerous filter and policy daemons), each running with reduced privileges and in a chroot jail for security. Key features include: SMTP server (smtpd) with configurable access control via smtpd_recipient_restrictions, smtpd_client_restrictions, and smtpd_sender_restrictions, support for SMTP AUTH (SASL via Dovecot or Cyrus), TLS encryption (via OpenSSL with opportunistic and mandatory TLS modes, DANE support, and certificate verification), content filtering integration (pipe interface for external spam/virus scanners like SpamAssassin, Amavisd, and Rspamd, and milter protocol support for Sendmail-compatible filters), address rewriting and masquerading, virtual domain hosting (separate virtual_alias_maps and virtual_mailbox_maps for unlimited domains and users without system accounts), mailbox delivery via local, virtual, or external LDA (Local Delivery Agent like Dovecot's lda or lmtp), relayhost configuration for smart host forwarding, policy delegation (delegating policy decisions to external policy servers via TCP), real-time blackhole list (RBL/DNSBL) checking, and comprehensive queue management (active, deferred, hold, incoming, maildrop queues with configurable timeouts and retry intervals).