qmail is a secure, reliable, efficient, simple message transfer agent.It is meant as a replacement for the entire sendmail-binmail system ontypical Internet-connected UNIX hosts.Secure: Security isn't just a goal, but an absolute requirement. Maildelivery is critical for users; it cannot be turned off, so it must becompletely secure. (This is why I started writing qmail: I was sick ofthe security holes in sendmail and other MTAs.)Reliable: qmail's straight-paper-path philosophy guarantees that amessage, once accepted into the system, will never be lost. qmail alsosupports maildir, a new, super-reliable user mailbox format. Maildirs,unlike mbox files and mh folders, won't be corrupted if the systemcrashes during delivery. Even better, not only can a user safely readhis mail over NFS, but any number of NFS clients can deliver mail to himat the same time.Efficient: On a Pentium under BSD/OS, qmail can easily sustain 200000local messages per day---that's separate messages injected and deliveredto mailboxes in a real test! Although remote deliveries are inherentlylimited by the slowness of DNS and SMTP, qmail overlaps 20 simultaneousdeliveries by default, so it zooms quickly through mailing lists. (Thisis why I finished qmail: I had to get a big mailing list set up.)Simple: qmail is vastly smaller than any other Internet MTA. Somereasons why: (1) Other MTAs have separate forwarding, aliasing, andmailing list mechanisms. qmail has one simple forwarding mechanism thatlets users handle their own mailing lists. (2) Other MTAs offer aspectrum of delivery modes, from fast+unsafe to slow+queued. qmail-sendis instantly triggered by new items in the queue, so the qmail systemhas just one delivery mode: fast+queued. (3) Other MTAs include, ineffect, a specialized version of inetd that watches the load average.qmail's design inherently limits the machine load, so qmail-smtpd cansafely run from your system's inetd.Replacement for sendmail: qmail supports host and user masquerading,full host hiding, virtual domains, null clients, list-owner rewriting,relay control, double-bounce recording, arbitrary RFC 822 address lists,cross-host mailing list loop detection, per-recipient checkpointing,downed host backoffs, independent message retry schedules, etc. Inshort, it's up to speed on modern MTA features. qmail also includes adrop-in ``sendmail'' wrapper so that it will be used transparently byyour current UAs.