The file on a Unix system that stores (you can probably guess where this is going, given the node title) one's incoming e-mail. In keeping with Unix tradition, mailbox files have a simple format: The messages are simply concatenated together, each one beginning (that is, separated from its predecessor) by a "From line" recapitulating the message's sender and the date it was received. Everything else in the file is just the individual messages, more or less exactly as they were received over the wire. These days, that means they're RFC822-format messages, with additional header lines followed by the body text.

To burst a mailbox file into individual messages, all you have to do is search for lines beginning with the five characters "From ", preceded by a blank line. Each such line demarcates a new message. (The software that built the file was careful to quote any message text lines beginning with "From " somehow, so that they won't confuse the bursting algorithm.)

Once upon a time, your mailbox was simply a file in this format, stored on your local system, usually in /usr/spool/mail or /var/spool/mail. Your mailreader simply interpreted the file for you (implicitly breaking it up, as described here, and presenting it as individual messages), or if you wanted to, you could peek at the raw file yourself, using cat or more or the text-manipulation utility of your choosing. More and more, however, incoming mail is collected on dedicated mail servers, and is accessed using protocols like POP or IMAP.