One of the reasons qmail is used is that it queues outgoing emails for
later delivery
If the system had to process multiple requests continuously
for outgoing mail such as requests to be removed from mailing lists, the server would soon be
overloaded and crash causing multiple problems.
This is particularly important to merchants who rely on
mass mailings in order to do
business.
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Qmail is easier to configure, a whole lot more secure (from a design
standpoint, I neither know nor care how many holes are actually left in sendmail),
and allows users to admin their own lists (which is quite useful). It, however,
is not a sendmail replacement.
When it was released, qmail was
significantly faster than Sendmail, particularly for bulk mail tasks such as mailing list servers.
qmail was originally designed as a way for managing large mailing
lists.