MBOXRD is one of the MBOX family of related file formats used for holding collections of electronic mail messages. See MBOX for a description of the syntax.
MBOXRD was designed to solve the message corruption problems inherent in the MBOXO format. It employs "reversible From quoting" because it allows for differentiating between ">From " found in the original message from ">From " found in a transformed MBOXO file. This is important because lines that begin with ">From " act as the separator lines between email messages; if ">From " appears in any line in original text outside of the start of a new message, it will become a misplaced separator line and incorrectly mark a new message when indeed, there is no new message.
In MBOXRD files, any line of the message, in either the header or the body, that begins with zero or more '>' characters followed by the five characters 'F', 'r', 'o', 'm', and ' ', has a single greater-than sign (">") character prepended to it. When a message is read from a MBOXRD mailbox file, it is transformed back so that any line of the message, in either the header or the body, that begins with one or more '>' characters followed by the five characters 'F', 'r', 'o', 'm', and ' ', has the single leading '>' character removed from it.
Because "From " at the start of a line is more probable than ">From " in real-world messages, an MBOXRD reader will restore a greater number of messages written to a mailbox by an MBOXO tool to their original forms than an MBOXRD tool, but will not and cannot restore all messages.