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qmail by John Levine

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Chapter 2. How Qmail Works

People who are familiar with other mail transfer agents (MTAs), notably sendmail, rarely receive satisfactory results from qmail. Qmail was designed and written in a very different way from most other mail programs, so approaches used to solve problems with other programs don’t work with qmail and vice versa.

Small Programs Work Together

Earlier MTAs were written as large monolithic programs. Sendmail, for example, is one large executable program that listens for incoming SMTP connections, accepts locally generated mail, queues mail, attempts outgoing SMTP deliveries, performs local deliveries, interprets .forward files, retries mail that for which earlier delivery attempts failed, and about 50 other functions. While this means that all of these functions can share utility routines and it’s easy for one function to call on another or pass a message to another, it also means that sendmail is a large program (300 KB of code on my system, not including any libraries it uses) that is slow to start up and expensive to fork, and bugs anywhere in the code can potentially make any of the functions misbehave or fail. Other monolithic MTAs, such as smail and exim, share these problems.

Qmail, on the other hand, is about 10 small programs, none with as much as 30 KB of code, working together. This design approach offers many advantages.

Each Program Does One Thing

Each of qmail’s programs performs a single function. For example, qmail-lspawn spawns (starts up) local ...

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