Sieve is a programming language that can be used for email filtering. It owes its creation to the CMU Cyrus Project, creators of Cyrus IMAP server.

The language is not tied to any particular operating system or mail architecture. It requires the use of RFC-2822–compliant messages, but otherwise generalizes to other systems that meet these criteria. The current version of Sieve's base specification is outlined in RFC 5228, published in January 2008.

Language
Sieve is a data-driven programming language, similar to earlier email filtering languages such as procmail and maildrop, and earlier line-oriented languages such as sed and AWK: it specifies conditions to match and actions to take on matching.

This differs from general-purpose programming languages while this is highly limited – the base standard has no variables, and no loops (but does allow conditional branching), preventing runaway programs which limits the language to simple filtering operations. Although extensions have been devised to extend the language to include variables and, limited loops, the language is still highly restricted, and thus suitable for running user-devised programs as part of the mail system.

There are also a significant number of restrictions on the grammar of the language, in order to reduce the complexity of parsing the language, but the language also supports the use of multiple methods for comparing localized strings, and is fully Unicode-aware.

While Sieve was originally conceived as tool external to SMTP,  serendipitously extends it in order to allow rejection at the SMTP protocol level.

Use
The Sieve scripts may be generated by a GUI-based rules editor or they may be entered directly using a text editor.

The scripts are transferred to the mail server in a server-dependent way. The ManageSieve protocol (defined in ) allows users to manage their Sieve scripts on a remote server. Mail servers with local users may allow the scripts to be stored in e.g. a  file in the users' home directories.

History 
The language was standardized in the (now-obsolete)  of January 2001, by Tim Showalter.

Extensions
The IETF Sieve working group has updated the base specification in 2008 (), and has brought the following extensions to Proposed Standard status:
  – Copying without side effects; allows a script to be copied across mailboxes without halting the script.
  – Body; allows a script to test the body of a message, not just its header.
  – Environment; provides access to information about the Sieve interpreter and its system environment.
  – Variables; allows the script to save and retrieve values in variables.
  – Vacation; specifies an action to send a response informing the sender that the recipient may be away.
  – Relational tests; defines numeric tests, so that a script may test a field for a numeric value, and may test against the number of occurrences of a field.
  – IMAP4flags; allows a script to test and set a message's IMAP flags.
  – Subaddress; allows a script to test subaddresses of the form "user+detail@domain.example".
  – Spamtest and Virustest; allows a script to interface with implementation
  – Date and Index Extensions.
  – Editheader; allows a script to add and delete message header fields.
  – Reject; allows messages to be rejected at either the LMTP/SMTP level or with an MDN or DSN.
  – Notifications; allows a script to trigger external notifications of email.
  – Notifications; allows notifications to be sent via the mailto protocol
  – XMPP notifications; specifies notifications via XMPP.
  – Checking; Mailbox status and accessing mailbox metadata.
  – MIME Part Tests, Iteration, Extraction, Replacement, and Enclosure
  – A Protocol for Remotely Managing Sieve Scripts
  – Vacation; adds the ability to specify seconds in a vacation time period.
  – MIME part conversion; allows a script to convert body parts between MIME types.
  – Script inclusion; allow a user to include one script inside another.
A number of other extensions are still being developed by the Sieve working group.

Example 
This is an example sieve script:
# Sieve filter

# Declare the extensions used by this script.
#
require ["fileinto", "reject"];

# Messages bigger than 100K will be rejected with an error message
#
if size :over 100K {
   reject "I'm sorry, I do not accept mail over 100kb in size. 
Please upload larger files to a server and send me a link.
Thanks.";
}

# Mails from a mailing list will be put into the folder "mailinglist" 
#
elsif address :is ["From", "To"] "mailinglist@blafasel.invalid" {
   fileinto "INBOX.mailinglist";
}

# Spam Rule: Message does not contain my address in To, CC or BCC
# header, or subject is something with "money" or "Viagra".
#
elsif anyof (not address :all :contains ["To", "Cc", "Bcc"] "me@blafasel.invalid", 
header :matches "Subject" ["*money*","*Viagra*"]) {
      fileinto "INBOX.spam";
}

# Keep the rest.
# This is not necessary because there is an "implicit keep" rule
#
else {
     keep;
}

See also 
 Email filtering
 procmail
 maildrop
 fdm

References

External links
 Sieve.Info, a Wiki Site about Sieve
 The old Sieve Home Page on web.archive.org
 
 Sieve IETF Working Group Charter
 Sieve tutorial with many examples

Email
Spam filtering
Request for Comments
Programming_languages_created_in_2001