Internet Message Access Protocol (IMAP)

This document extends Internet Message Access Protocol (IMAP) with a new capability called "STATUS=SIZE". To determine the total storage size of a mailbox, an IMAP client currently needs to retrieve all message sizes individually using the FETCH command with the RFC822.SIZE data item. The "STATUS=SIZE" capability provides a more efficient means to achieve this. It extends the STATUS command with a new "SIZE" data item, which indicates the total size of all messages in the target mailbox. This way, this information can be queried with just one STATUS command. When the "LIST-STATUS" IMAP capability is also available, the SIZE data item can be queried for many mailboxes at once using just one LIST command. This capability is particularly useful for IMAP clients that do not cache the state of the message store, such as most webmail clients. Without the "STATUS=SIZE" capability, such clients need to fetch all message sizes from the server when the size of a mailbox needs to be determined, for example when the user requests detailed quota usage information. The "QUOTA" capability is no help in that scenario, since the provided QUOTAROOT command can only yield the "STORAGE" resource usage of a whole quota root, but not for a single mailbox within that root.

In examples, "C:" indicates lines sent by a client that is connected to a server. "S:" indicates lines sent by the server to the client. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

This extension defines one new status data item for the STATUS command and response: The total size of the mailbox in octets. This MUST be equal to the sum of the values of the RFC822.SIZE FETCH message data item of all messages in the mailbox. Example:

The same information can be obtained by using the following commands, albeit less efficient:

When the LIST-STATUS IMAP capability is also available, the STATUS command can be combined with the LIST command to further improve efficiency. This way, the sizes of many mailboxes can be queried with just one LIST command. Example:

The following syntax specification augments the grammar specified in . It uses the Augmented Backus-Naur Form (ABNF) notation as specified in . Elements not defined here are taken from .

[FIXME: Interaction with 64BIT capability]

There are no known additional security issues with this extension beyond those described in the base protocol described in .

The IANA is requested to add "STATUS=SIZE" to the "IMAP Capabilities" registry located at <http://www.iana.org/assignments/imap-capabilities>.

Thanks to Alexey Melnikov and Michael Slusarz for reviews and suggestions. More reviews are appreciated.