qmail VS Diffee

• Strong security record
  qmail was designed by Daniel J. Bernstein with security as the primary goal. It is split into mutually untrusting components that run with minimal privileges, and it avoids risky C library patterns. Its security record is excellent, and Bernstein offered a public bounty for years that went unclaimed for a verified security hole in the core design.
• Modular architecture
  The system is broken into small programs (qmail-smtpd, qmail-send, qmail-queue, qmail-local, qmail-remote and others), each with a narrow job. This makes the code easier to audit, limits the damage of any single bug, and lets administrators replace or extend individual pieces.
• Reliable queue and delivery
  qmail uses a robust queue design and Maildir delivery, which avoids file locking and mailbox corruption problems. Messages are written safely to disk before acceptance is acknowledged, which reduces the risk of lost mail even after crashes or power failures.
• High performance and efficiency
  qmail uses massive parallel delivery, with configurable concurrency, and has a small resource footprint. It was much faster than Sendmail in its day and handles large volumes of mail, including mailing lists, well on modest hardware.
• Simple configuration
  Configuration is done through small plain-text files in the control directory instead of a large, complicated config language like Sendmail's. Behavior is generally easy to understand and predictable, and the Maildir and .qmail forwarding files give users flexible per-user delivery control.

Possible disadvantages

• Abandoned core development
  The original qmail 1.03 was released in 1998 and has not been updated by the author. It lacks modern features and fixes, so administrators depend on third-party patches or forks such as netqmail, notqmail, or s/qmail to remain current.
• Lacks modern features out of the box
  The stock distribution has no built-in support for TLS/STARTTLS, SMTP AUTH, IPv6, spam or virus filtering, SPF, DKIM, or DMARC. Each of these requires unofficial patches or add-ons, which makes a modern deployment complicated.
• Patching complexity and maintenance burden
  Because so many capabilities depend on community patches that may conflict with each other, building a fully featured qmail server is laborious. Combining patches requires manual merging and testing, and there is no unified upstream to rely on for security updates.
• Unconventional licensing and distribution terms
  qmail was not open source under a standard license for many years. Redistributing modified binaries was restricted, which hampered packaging by Linux distributions and led to source-only install procedures. It was placed in the public domain in 2007, but the legacy of that restriction still affects its ecosystem.
• Steep learning curve and unconventional design
  qmail's layout departs from traditional Unix mail conventions, with its own directory structure, the need for daemontools or ucspi-tcp, and different terminology. Administrators familiar with Postfix or Sendmail must learn new tools, and documentation and community support have declined over time.

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