The POPular POP server uses a slightly modified Maildir format for storing mailboxes. The format is for most uses compatible to the original mailbox format, allowing standard software components to be used.
In contrast to the original Maildir format the line ending used is CRLF and not only LF. Both SMTP and POP3 use CRLF as a line ending and there is no sense converting to LF endings when saving the mail and back to CRLF when reading the mail. The added advantage is that the size of the file is exactly the file size that is reported in POP3 or with the ESMTP SIZE extension in SMTP.
With the Maildir format each mail in a mailbox is saved in its own file instead of all mails together in one file. A mailbox is a directory with the three subdirectories new, cur and tmp. New, unread mails are in the new directory. Read mails are in the cur directory. The tmp is used for delivery. A mail is saved in this directory with a unique name. Only after the mail has been written completely to disk it is moved to the new directory. With this scheme no locking or copying of mails or mailboxes is needed when delivering, reading or deleting mails.
The filenames used for the mails must be unique. Somewhat differently than in the original description of the Maildir format the following naming scheme is used: A filename has three components, the UNIX system time (in seconds after 1970/1/1), the process id of the process delivering the mail and the size of the mail. The components are separated by a dot (.) and an underscore (_), respectively. An example for a file name would be 945113196.14341_1067. The size of the mail is saved in the filename, so that we don't have to stat the file in order to find out about its size.
With the original Maildir format, the hostname of the delivering system is written into the filename. This is only needed if NFS is used for mailbox access or mail files are copied between hosts to the same mailbox. Because we don't do any of this, we don't put the hostname in there.
A Status-Header is never saved inside a mail message file, because
it would mean that the file has to be rewritten after it is RETRieved,
but not DELEted. Instead, pserv will add a
Status-Header on the fly while sending the mail out if the
statusheader configuration variable is set and
the mail file is in the cur directory.
POPular can cope with several pathologic cases of mail file contents like totally empty mails, mails without headers etc. Empty mails will not be accessible, an error is logged. Other strange mails are accessible, but the client might not understand it. Mails that don't end in CRLF have a CRLF appended, so that the end-of-file-marker 'CRLF.CRLF' is recognized. The appended CRLF don't count towards the mail size.