-- Ernie P.
qmail is a secure, reliable, efficient, simple message transfer agent. It is
meant as a replacement for the entire sendmail-binmail system on typical
Internet-connected UNIX hosts.
Secure: Security isn't just a goal, but an absolute requirement. Mail
delivery is critical for users; it cannot be turned off, so it must be
completely secure. (This is why I started writing qmail: I was sick of the
security holes in sendmail and other MTAs.)
Reliable: qmail's straight-paper-path philosophy guarantees that a message,
once accepted into the system, will never be lost. qmail also supports
maildir, a new, super-reliable user mailbox format. Maildirs, unlike mbox
files and mh folders, won't be corrupted if the system crashes during
delivery. Even better, not only can a user safely read his mail over NFS, but
any number of NFS clients can deliver mail to him at the same time.
Efficient: On a Pentium under BSD/OS, qmail can easily sustain 200000 local
messages per day---that's separate messages injected and delivered to
mailboxes in a real test! Although remote deliveries are inherently limited
by the slowness of DNS and SMTP, qmail overlaps 20 simultaneous deliveries by
default, so it zooms quickly through mailing lists. (This is why I finished
qmail: I had to get a big mailing list set up.)
Simple: qmail is vastly smaller than any other Internet MTA. Some reasons
why: (1) Other MTAs have separate forwarding, aliasing, and mailing list
mechanisms. qmail has one simple forwarding mechanism that lets users handle
their own mailing lists. (2) Other MTAs offer a spectrum of delivery modes,
from fast+unsafe to slow+queued. qmail-send is instantly triggered by new
items in the queue, so the qmail system has just one delivery mode:
fast+queued. (3) Other MTAs include, in effect, a specialized version of
inetd that watches the load average. qmail's design inherently limits the
machine load, so qmail-smtpd can safely 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. In short, it's up to
speed on modern MTA features. qmail also includes a drop-in ``sendmail''
wrapper so that it will be used transparently by your current UAs.
ftp://koobera.math.uic.edu/www/proto/design.html
Protocol design
So you want to design a protocol? I'll let you in on an evil secret: most
implementors will never read your spec.
Instead, they'll look at some common examples and write the most obvious
code that works for those examples.
Maybe you don't like this. Maybe you don't think implementors should be
passing off prototypes as finished products. Tough luck. Do you want to
complain, or do you want to design a protocol that works?
Here are a few case studies....