Mbox

mbox is a single file format for storing mail messages. It is often considered to have been made obsolete by maildir. Because it stores all the messages in one file, there is a possibility of corruption when two different programs try and edit the file at the same time.

The format of the file is simple. Each message starts with the five characters "From " and ends with an extra blank line at the bottom. This lets mail programs easily find the start of the next message by looking for those five characters preceded by a newline. Little else is done to format or store the message other than dumping the raw contents into the file. There is one exception to the general rule of not changing a message. If the message has a line which starts with "From " in the body, it will usually add a character in front of that, typically ">", to prevent the mail clients, MUA, from becoming confused and thinking a new message is starting.