mboxrd
mboxrd is a mailbox file format, a variant of the classic mbox format used to store multiple email messages in a single text file. In mbox-style mailboxes, messages are stored sequentially, and each message begins with a line that starts with From followed by the envelope sender and the date. This line serves as a delimiter that separates messages within the file. The remainder of the content for that message consists of the headers and the body, up to the next delimiter line.
mboxrd differs from some other mbox variants in how it handles literal lines that could be mistaken
Compatibility and usage: mboxrd has been implemented by various Unix mail programs and libraries, including parts