Exim Mail Transfer Agent

Exim is a highly configurable message transfer agent (MTA) developed at the University of Cambridge by Philip Hazel in 1995, widely deployed on Unix-like systems for email routing and delivery. Known for its extraordinary flexibility, Exim powers significant portions of internet email infrastructure. Key features include: policy-based routing (exim.conf configuration with ACLs, routers, and transports for per-domain and per-recipient message routing), address routing (multiple router types including DNS lookup, manualroute, redirect, accept, and iplookup for complex routing scenarios), delivery transports (SMTP remote delivery, appendfile for local Maildir and mbox delivery, pipe for external program delivery, and autoreply for vacation messages), access control lists (ACLs for SMTP commands including HELO, MAIL FROM, RCPT TO, DATA, and MIME with regex and database lookups for spam and virus filtering), authentication (PLAIN, LOGIN, CRAM-MD5, and SPA authentication with LDAP, SQL, and PAM backend support), TLS and SSL (STARTTLS and SMTPS support with certificate management and OCSP stapling), content scanning (built-in malware scanning via clamav, spamassassin integration, and MIME part extraction for attachment filtering), database lookups (MySQL, PostgreSQL, Oracle, LDAP, Redis, cdb, and DBM for dynamic configuration), mail queue management (sophisticated queue management with retry rules, queue runners, and frozen message handling), and real-time blocklist support (DNSBL and RHSBL lookups for IP and domain reputation checking at SMTP time).