The traditional unixlike system's email storage format; pioneered by the ubiquitous (and somewhat evil) sendmail. An mbox is a single flat file containing all of a user's emails, concatenated front to back.
Mboxes are somewhat faster to read from (but slower to write to) than maildirs. More importantly, they are considerably more subject to potential corruption issues - on a heavily trafficked server, file locking has to be carefully implemented to keep multiple processes from clobbering a single mbox they all want to write to. Worse yet, a single corrupt file can result in losing ALL of a user's email; whereas with maildir there is no simple way (short of somebody typing rm -rf from a shell prompt) to lose more than one email with a single operation or corruption.
Contrast with maildir.
See also Postfix, an alternative mail server, that offers mbox capability.