mbox
Unix's oldest way of storing email: every message one after another in a single text file, separated by lines beginning with From .
The format dates from 1970s Unix and existed in practice long before anyone bothered to write it down. Mail delivery appended each new message to /var/mail/username, preceded by a line carrying the sender and date. A mail reader only had to look for lines starting with those five characters, From , to find the boundaries.
Simplicity has a price. Any line in a message body that happened to start with "From " had to be escaped as >From , and since programs did that differently, the variants mboxo, mboxrd, mboxcl and mboxcl2 emerged — and don't always understand each other. Two programs writing at once could corrupt the whole mailbox, making locking a perennial source of bugs. The format got its own RFC, 4155, only in 2005, and that document mostly concedes that there are lots of them.