Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
The subject of standard Unix and Linux mail file formats came up as part of a recent article I was writing and I decided that it was something that needed to be addressed to avoid confusion when talking about it in the future.
The two primary formats for mail storage in Linux and Unix are Mbox and MailDir. There are several other formats out there as well, including MMDF (a close cousin to Mbox), MH (similar to MailDir, but used more in IMAP mail senarios), and BABYL. Other than Mbox and MailDir, most other formats are either deprecated, obsolete, or in very limited use. Hence most end users won't encounter said formats and thus don't need to worry about them. Therefore, we'll only concentrate today on the primary two formats.