Chapter 11. In depth technicalities

Each backend is a pplistener daemon. A pplistener stores all messages under one doubly hashed directory, so a typical Maildir might be '/var/powermail/messages/01/04/info@company.biz/'. Because all mail is stored in a single filesystem, it is possible for a file to be present in many mailboxes, and this is in fact how PowerMail operates.

The operating system will only actually store one (1) copy of a message, no matter how many recipients that message has on that filesystem. It will be deleted when it is removed from the last mailbox that references the message.

This allows for great space savings and, more importantly, for 'straight paper path' delivery.

11.1. Maildir and queue-less delivery

Each mailbox is more-or-less a standard Qmail Maildir, which means that new messages are written in the 'tmp' subdirectory and are moved to 'cur' subdirectory then they are ready.

Because of this, no locking is needed, and messages can be delivered atomically to many recipients, with intermediate failure at worst causing a duplicate delivery.

Because of the use of hardlinks, it is possible to deliver messages straight into the Maildir, without use of a queue in between. This makes mail delivery very fast and robust.