Sifter3 - Sieve email filter (RFC 5228)

Sifter3 is a Python 3 implementation of the Sieve email filter language (RFC 5228)


  • Supports all of the base Sieve spec from RFC 5228, except for features still listed under TODO below
    • multiline strings (since version 0.2.2)
    • bracketed comments (since version 0.2.4)
  • Extensions supported:
    • regex (draft-ietf-sieve-regex-01)
    • body (RFC 5173)
    • variables (RFC 5229)
    • enotify (RFC 5435, particularly the mailto method RFC 5436)
    • imap4flags (RFC 5232: setflag, addflag, removeflag; not supported: hasflags, :flags)
    • reject and ereject (RFC 5429) (since version 0.2.4)
    • ihave (RFC 5463) (since version 0.2.5)


pip install sifter3


import email
import sifter.parser
rules = sifter.parser.parse_file(open('my_rules.sieve'))
msg = email.message_from_file(open('an_email_to_me.eml'))
msg_actions = rules.evaluate(msg)

In the above example, msg_actions is a list of actions to apply to the email message. Each action is a tuple consisting of the action name and action-specific arguments. It is up to the caller to manipulate the message and message store based on the actions returned.


The output of the command line tool can be parsed as json.

$ sifter tests/evaluation_1.rules tests/evaluation_1.msg
[['redirect', '']]


  • No thought has been given yet to hardening against malicious user input. The current implementation is aimed at users that are running their own sieve scripts.
  • The current implementation is not optimized for performance, though hopefully it’s not too slow for normal inputs.


  • An example adaptor that provides Unix LDA behavior using sieve for filtering
  • Base spec features not yet implemented:
    • encoded characters (section
    • message uniqueness (section 2.10.3)
    • envelope test (section 5.4)
    • handle message loops (section 10)
    • limit abuse of redirect action (section
    • address test should limit allowed headers to those that contain addresses (section 5.1)