Sieve Email Filtering: Environment Extension
RFC 5183

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Network Working Group                                           N. Freed
Request for Comments: 5183                              Sun Microsystems
Category: Standards Track                                       May 2008

              Sieve Email Filtering: Environment Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes the "environment" extension to the Sieve
   email filtering language.  The "environment" extension gives a Sieve
   script access to information about the Sieve interpreter itself,
   where it is running, and about any transport connection currently
   involved in transferring the message.

1.  Introduction

   Sieve [RFC5228] is a language for filtering email messages at or
   around the time of final delivery.  It is designed to be
   implementable on either a mail client or mail server.  It is suitable
   for running on a mail server where users may not be allowed to
   execute arbitrary programs, such as on black box Internet Message
   Access Protocol [RFC3501] servers, as it has no user-controlled loops
   or the ability to run external programs.

   Although Sieve is intended to be independent of access protocol, mail
   architecture, and operating system, in some cases it is useful to
   allow scripts to access information about their execution context.
   The "environment" extension provides a new environment test that can
   be used to implement scripts that behave differently when moved from
   one system to another, when messages arrive from different remote
   sources or when otherwise operated in different contexts.

2.  Conventions Used in This Document

   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 [RFC2119].

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RFC 5183              Sieve Environment Extension               May 2008

   The terms used to describe the various components of the Sieve
   language are taken from Section 1.1 of [RFC5228].

   This document refers to the ABNF productions IPv4-address-literal,
   IPv6-address-literal, and General-address-literal defined in Section
   4.1.3 of [RFC2821].

   The location item makes use of standard terms for email service
   components.  Additional information and background on these terms can
   be found in [EMAIL-ARCH].

3.  Capability Identifiers

   The capability string associated with the extension defined in this
   document is "environment".

4.  Environment Test

   Usage:   environment [COMPARATOR] [MATCH-TYPE]
                        <name: string>
                        <key-list: string-list>

   The environment test retrieves the item of environment information
   specified by the name string and matches it to the values specified
   in the key-list argument.  The test succeeds if a match occurs.  The
   type of match defaults to ":is" and the default comparator is
   "i;ascii-casemap".

   The current message is not a direct source of information for the
   environment test; the item of information specified by the name
   string is extracted from the script's operating environment and the
   key-list argument comes from the script.

   The environment test MUST fail unconditionally if the specified
   information item does not exist.  A script MUST NOT fail with an
   error if the item does not exist.  This allows scripts to be written
   that handle nonexistent items gracefully.  In particular, the test:

     if environment :contains "item" "" { ... }

   only succeeds if "item" is known to the implementation, and always
   succeeds if it is.

   The "relational" extension [RFC5231] adds a match type called
   ":count".  The count of an environment test is 0 if the environment
   information returned is the empty string, or 1 otherwise.

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RFC 5183              Sieve Environment Extension               May 2008

   Environment items can be standardized or vendor-defined.  An IANA
   registry is defined for both types of items.  Extensions designed for
   interoperable use SHOULD be defined in standards track or
   experimental RFCs.

4.1.  Initial Standard Environment Items

   The initial set of standardized environment items is as follows:

    "domain"  => The primary DNS domain associated with the Sieve
                 execution context, usually but not always a proper
                 suffix of the host name.

    "host"    => The fully-qualified domain name of the host where