Display-based Address Sorting for the IMAP4 SORT Extension

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 .

The extension to the protocol provides a means for server-based sorting of messages. It defines a set of sort criteria and the mechanism for determining the sort value of a message for each such ordering. The extension's FROM ordering sorts messages lexically on the addr-part of the first address in the From header. This document provides an alternate ordering, DISPLAYFROM, which sorts messages lexically based on the From address's display-name, when present. A server that supports the DISPLAYFROM sort criterion indicates this by returning "SORT=DISPLAYFROM" in its CAPABILITY response.

This document introduces a new sort-key, DISPLAYFROM. A message's sort value under the DISPLAYFROM ordering MUST be derived from the first mailbox in its From header as follows: If the From header is absent or if the header contains no mailboxes, the message's sort value is the empty string. If the mailbox contains an display-name, replace each instance of CFWS in the display-name with a single space, trim all leading and trailing whitespace, and decode any encoded-words. If the resulting string is not the empty string, use it as the sort value for the message. Otherwise, completely remove all CFWS from the mailbox's addr-spec and use the resulting string as the message's sort value.

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in . defines the non-terminal "capability" and defines "sort-key".

This document defines an additional IMAP4 capability. As such, it does not change the underlying security considerations of . The author believes that no new security issues are introduced with this additional IMAP4 capability.

DISPLAYFROM is a string-based sort criterion. As stated in , the collations mandated by I18NLEVEL=1 or I18NLEVEL=2 from MUST be followed when sorting such strings. The DISPLAYFROM ordering sorts on the full decoded display-name, when present. It does not attempt to parse this string in a locale- or language-dependent manner in order to determine and sort on some semantically meaningful substring such as the surname.

capabilities are registered by publishing a standards track or IESG-approved experimental RFC. This document constitutes registration of the SORT=DISPLAYFROM capability in the capabilities registry.