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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)
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