Here are some of qmail's features. Setup:*  automatic adaptation to your UNIX variant---no configuration needed*  AIX, BSD/OS, FreeBSD, HP/UX, Irix, Linux, OSF/1, SunOS, Solaris, and more*  automatic per-host configuration (config, config-fast)*  quick installation---no big list of decisions to makeSecurity:*  clear separation between addresses, files, and programs*  minimization of setuid code (qmail-queue)*  minimization of root code (qmail-start, qmail-lspawn)*  five-way trust partitioning---security in depth*  optional logging of one-way hashes, entire contents, etc. (QUEUE_EXTRA)Message construction (qmail-inject):*  RFC 822, RFC 1123*  full support for address groups*  automatic conversion of old-style address lists to RFC 822 format*  sendmail hook for compatibility with current user agents*  header line length limited only by memory*  host masquerading (control/defaulthost)*  user masquerading ($MAILUSER, $MAILHOST)*  automatic Mail-Followup-To creation ($QMAILMFTFILE)SMTP service (qmail-smtpd):*  RFC 821, RFC 1123, RFC 1651, RFC 1652, RFC 1854*  8-bit clean*  931/1413/ident/TAP callback (tcp-env)*  relay control---stop unauthorized relaying by outsiders (control/rcpthosts)*  no interference between relay control and forwarding*  tcpd hook---reject SMTP connections from known abusers*  automatic recognition of local IP addresses*  per-buffer timeouts*  hop countingQueue management (qmail-send):*  instant handling of messages added to queue*  parallelism limit (control/concurrencyremote, control/concurrencylocal)*  split queue directory---no slowdown when queue gets big*  quadratic retry schedule---old messages tried less often*  independent message retry schedules*  automatic safe queueing---no loss of mail if system crashes*  automatic per-recipient checkpointing*  automatic queue cleanups (qmail-clean)*  queue viewing (qmail-qread)*  detailed delivery statistics (qmailanalog, available separately)Bounces (qmail-send):*  QSBMF bounce messages---both machine-readable and human-readable*  HCMSSC support---language-independent RFC 1893 error codes*  double bounces sent to postmasterRouting by domain (qmail-send):*  any number of names for local host (control/locals)*  any number of virtual domains (control/virtualdomains)*  domain wildcards (control/virtualdomains)*  configurable percent hack support (control/percenthack)*  UUCP hookSMTP delivery (qmail-remote):*  RFC 821, RFC 974, RFC 1123*  8-bit clean*  automatic downed host backoffs*  artificial routing---smarthost, localnet, mailertable (control/smtproutes)*  per-buffer timeouts*  passive SMTP queue---perfect for SLIP/PPP (serialmail, available separately)Forwarding and mailing lists (qmail-local):*  address wildcards (.qmail-default, .qmail-foo-default, etc.)*  sendmail .forward compatibility (dot-forward, available separately)*  fast forwarding databases (fastforward, available separately)*  sendmail /etc/aliases compatibility (fastforward/newaliases)*  mailing list owners---automatically divert bounces and vacation messages*  VERPs---automatic recipient identification for mailing list bounces*  Delivered-To---automatic loop prevention, even across hosts*  automatic mailing list management (ezmlm, available separately)Local delivery (qmail-local):*  user-controlled address hierarchy---fred controls fred-anything*  mbox delivery*  reliable NFS delivery (maildir)*  user-controlled program delivery: procmail etc. (qmail-command)*  optional new-mail notification (qbiff)*  optional NRUDT return receipts (qreceipt)*  conditional filtering (condredirect, bouncesaying)POP3 service (qmail-popup, qmail-pop3d):*  RFC 1939*  UIDL support*  TOP support*  APOP hook*  modular password checking (checkpassword, available separately)