In addressing these different requirements, several ways of storing email (storage formats) have become popular. One of the oldest and least well defined is known as mbox. An mbox-formatted mailbox is a single file containing several messages concatenated together. There are several varieties of mbox formats, and the distinctions between them are primarily in the way each email is distinguished from the next, though there are other differences. One of the biggest drawbacks of this approach is that modifying or deleting messages that are stored in the middle (or at the beginning) of a large mailbox requires rewriting the entire mbox file. If the mailbox is large, this can take a significant amount of time. Another drawback of the mbox format is that the mailbox file must be locked before it can be modified. In some circumstances, like when the mbox is stored on an NFS drive, locking can be unreliable. In other circumstances, such as high-volume mailboxes, forcing all...