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 make

Security:

*  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 counting

Queue 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 postmaster

Routing 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 hook

SMTP 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)