Qmail is an Internet Mail
Transfer Agent (MTA) for UNIX-like operating systems. It's a
drop-in replacement for the Sendmail system provided with
UNIX operating systems. Qmail uses the Simple Mail Transfer
Protocol (SMTP) to exchange messages with MTA's on other
systems.
Why use Qmail?
Your operating system included an MTA, probably Sendmail, so
if you're reading this document you're probably looking for
something better. Some of the advantages of qmail over
vendor-provided MTA's include:
Security
qmail was designed for high security. Sendmail has a long
history of serious security problems. When Sendmail was
written, the Net was a much friendlier place. Everyone knew
everyone else, and there was little need to design and code
for high security. Today's Internet is a much more hostile
environment for network servers. Sendmail's author, Eric
Allman, and the current maintainer, Claus Assman, have done
a good job of tightening up the program, but nothing short
of a redesign can achieve true security.
Performance
qmail parallelizes mail delivery, performing up to 20
deliveries simultaneously, by default.
Reliability
Once qmail accepts a message, it guarantees that it won't be
lost. qmail also supports a new mailbox format that works
reliably even over NFS without locking.
Simplicity
qmail is smaller than any other equivalently-featured MTA.
