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A “maildir” is a simple directory structure originally developed for qmail that allows safe atomic update without locking. Create a base directory with three subdirectories: “new”, “tmp”, and “cur”. When messages arrive, they are put into a uniquely-named file (using pids, timestamps, and random numbers) in “tmp”. When the file is complete, it is atomically renamed into “new”. Eventually the buildmaster notices the file in “new”, reads and parses the contents, then moves it into “cur”. A cronjob can be used to delete files in “cur” at leisure.

Maildirs are frequently created with the maildirmake tool, but a simple mkdir -p ~/MAILDIR/{cur,new,tmp} is pretty much equivalent.

Many modern MTAs can deliver directly to maildirs. The usual .forward or .procmailrc syntax is to name the base directory with a trailing slash, so something like ~/MAILDIR/ . qmail and postfix are maildir-capable MTAs, and procmail is a maildir-capable MDA (Mail Delivery Agent).

For MTAs which cannot put files into maildirs directly, the “safecat” tool can be executed from a .forward file to accomplish the same thing.

The Buildmaster uses the linux DNotify facility to receive immediate notification when the maildir's “new” directory has changed. When this facility is not available, it polls the directory for new messages, every 10 seconds by default.