Simple Mail Transfer Protocol SMTP 1982-2026 (差別化: 既存Mail Server Postfix/Mailcow比較と異なりSMTP規格起源軸)。Ray Tomlinson 1971 BBN ARPANET 初Email送信 (@記号使用・QWERTYUIOP本文)・Jon Postel USC ISI 1982/8 RFC 821「Simple Mail Transfer Protocol」+RFC 822「Standard for the Format of ARPA Internet Text Messages」発表 (Header規格・「@」アドレス標準化)・Eric Allman UC Berkeley Sendmail 1981/3 (V8 Sendmail・Sendmail Inc 1998・Maintenance継続・mc/m4 Configuration複雑悪名)・Wietse Venema Postfix 1998 (IBM Watson Research・Sendmail代替・セキュリティ強化・モジュラー設計)・qmail Daniel J. Bernstein DJB 1995 (BSD-style License)+Exim Philip Hazel Cambridge 1995 (Debian/Ubuntu Default)・MX Mail Exchange Record DNS+SPF Sender Policy Framework RFC 7208+DKIM DomainKeys Identified Mail RFC 6376+DMARC RFC 7489+ARC RFC 8617・SMTP拡張ESMTP RFC 5321 2008+RFC 5322 (Internet Message Format Update)+8BITMIME・S/MIME+PGP Email暗号化+SMTPS Port 465 (TLS)+STARTTLS Port 587 Submission・¥0 標準, 2026年Postfix+Exim+Sendmail+Microsoft Exchange+Google Gmail主流。