to 1982 (RFC 822). That is what POP and IMAP clients expect to retrieve. Write access Messages sent to a mailbox are written by a mail delivery agent into the server's local mailbox, which, for remote users, is a remote mailbox that they own on that server. IMAP clients can copy, move, and delete messages in remote mailboxes. Size quota Mailboxes have a size limit, either determined implicitly by available memory, or after quota definitions for that mailbox or folders thereof. Besides administrative trivia, quota limits help mitigate email bomb attacks. An IMAP extension for quota was standardized in 1997. Storage format Any kind of database can be used to store email messages. However, some standardization has resulted in several well-known file formats to allow access to a given mailbox by different computer programs. There are two kinds of widely used formats: mbox is the original technique of storing all messages in a single file, Maildir is a newer specification that provides for storing all messages in a directory tree, with one file for each message. Mailbox names A mailbox name is the first part of an email address, also known as local-part; that is, the part before the @ symbol. Its format is formally specified by RFC 5322 and RFC 5321. It is often the username of the recipient on the mail server or in the destination domain. The local-part may be up to 64 characters long and, in theory, is case-sensitive. It can consist of either a sequence of valid characters (described below) or a quoted string, which can also contain spaces and special characters. Using SMTPUTF8 extension of SMTP it is also possible to use non-ASCII characters. Some common sense is needed when creating new mailbox names, in order to avoid common pitfalls. In the words of RFC 5321, very wary of imposing restrictions: Valid characters The following characters may appear in a local-part without quoting: Uppercase and lowercase English letters (a–z, A–Z), and UTF-8 sequences if using SMTPUTF8 Digits 0 to 9 Characters ! # $ % & ' * + - / = ? ^ _ ` { | } ~ Character . (dot) provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively (e.g. John..Doe@example.com). Reserved names The names "postmaster", "abuse", and others correspond to well-known roles and functions, and are