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This is the Trusted Domain Project's impementation of the DMARC protocol libary and mail filter, called OpenDMARC. A "milter" connects to unix-based mailers (originally, sendmail, but now many) and provides a standard filtering API.

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This directory has the latest open source DMARC software from The Trusted Domain Project.

There is a web site at that is home for the latest updates.

On GitHub, the "Master" branch follows the latest released version, while continued development happens on the "develop" branch.


The OpenDMARC project is a community effort to develop and maintain an open source package for providing DMARC report generation and policy enforcement services. It includes a library for handling DMARC record parsing, a database schema and tools for aggregating and processing transaction history to produce DMARC reports, and a filter that ties it all together with an MTA using the milter protocol.

In simple terms, DMARC takes the results of ARC, SPF and DKIM checks, done by either upstream filters, or SPF checks that opendmarc performs itself, and uses these to make a "pass or fail" decision. A domain owner may put a record in the DNS to determine what should happen to a failing record: No negative action (typically for testing), message quarantining, or outright rejection at SMTP acceptance time.

Additionally, records placed in the DNS allow a domain owner to receive reports back on when messages are received that fail DMARC, as well as specifying what percentage of messages should be evaluated.

This README is not intended to be a full explanation of how the DMARC protocol works, but at the very least, some software that does DKIM checks should be available in your mail stream in order to use this software.

The word "milter" is a portmanteau of "mail filter" and refers to a protocol and API for communicating mail traffic information between MTAs and mail filtering plug-in applications. It was originally invented at Sendmail, Inc. but has also been adapted to other MTAs.


To compile and operate, this package requires the following:

  • sendmail v8.13.0 (or later), or Postfix 2.3, (or later) and libmilter. (These are only required if you are building the filter.)

  • glib (GLib) headers and libraries 2.48.2 (or greater)

  • Access to a working nameserver (required only for signature verification).

  • A perl interpreter (required for sending or receiving and interpreting reports).

  • If you are interested in tinkering with the build and packaging structure, you may need to upgrade to these versions of GNU's "autotools" components:

    • autoconf (GNU Autoconf) 2.61
    • automake (GNU automake) 1.7 (or 1.9 to avoid warnings)
    • (GNU libtool) 2.2.6 (or 1.5.26 after make maintainer-clean)

Related Documentation

The man page for opendmarc (the actual filter program) is present in the opendmarc directory of this source distribution. There is additional information in the INSTALL and FEATURES files, and in the README file in the opendmarc directory. Changes are documented in the RELEASE_NOTES file.

HTML-style documentation for libopendmarc is available in libopendmarc/docs in this source distribution.

General information about DMARC can be found at

Mailing lists discussing and supporting the DMARC software found in this package are maintained via a list server at Visit to subscribe or browse archives. The available lists are:

  • opendmarc-announce (moderated) Release announcements.

  • opendmarc-users General OpenDMARC user questions and answers.

  • opendmarc-dev Chatter among OpenDMARC developers.

  • opendmarc-code Automated source code change announcements.

Bug tracking is done via the trackers on GitHub at:

You can enter new bug reports there, but please check first for older bugs already open, or even already closed, before opening a new issue.

Note that development is being moved away from SourceForge, Freshmeat, or other sites.

Directory Structure

  • contrib: A collection of user contributed scripts that may be useful.

  • db: Database schema and tools for generating DMARC reports based upon accumulated data.

  • libopendmarc:A library that implements the DMARC standard.

  • libopendmarc/docs: HTML documentation describing the API provided by libopendmarc.

  • opendmarc: A milter-based filter application which uses libopendmarc (and optionally libar) to provide DMARC service via an MTA using the milter protocol.

Runtime Issues

Missing symbols

You may receive the warning: WARNING: symbol 'X' not available

This indicates that the filter attempted to get some information from the MTA that the MTA did not provide.

At various points in the interaction between the MTA and the filter, certain macros containing information about the job in progress or the connection being handled are passed from the MTA to the filter.

In the case of sendmail, the names of the macros the MTA should pass to the filter are defined by the Milter.macros settings in, e.g.Milter.macros.connect, Milter.macros.envfrom, etc. This message indicates that the filter needed the contents of macro X, but that macro was not passed down from the MTA.

Typically the values needed by this filter are passed from the MTA if the was generated by the usual m4 method. If you do not have those options defined in your, make sure your M4 configuration files are current and rebuild your to get appropriate lines added to your, and then restart sendmail.

MTA timeouts

By default, the MTA is configured to wait up to ten seconds for a response from a filter before giving up. When querying remote nameservers for key and policy data, the DMARC filter may not get a response from the resolver within that time frame, and thus this MTA timeout will occur.

This can cause messages to be rejected, temp-failed or delivered without verification, depending on the failure mode selected for the filter.

When using the standard resolver library provided with your system, the DNS timeout cannot be adjusted. If you encounter this problem, you must increase the time the MTA waits for replies. See the documentation in the sendmail open source distribution (libmilter/README in particular) for instructions on changing these timeouts.

When using the provided asynchronous resolver library, you can use the -T command line option to change the timeout so that it is shorter than the MTA timeout.

Other OpenDMARC issues:

Bug tracking is done via the trackers on GitHub at:

Please report them there, after checking for prior reports.

Further Reading

As DMARC adoption becomes more common, any list of links placed in the README of a single implementation will invariably grow out of date. Using your favorite search engine, or the mailing lists for your operating system or MTA is not an unreasonable path forward.

As a start, however, the RFC's that define SPF, DKIM, and DMARC present a fairly comprehensive, if technical, understanding of the underlying protocols. Although there is not much information involving marrying them to a specific mail server.

At the time of this writing, the following are the most recent RFC's for the protocols involved (although many other RFC's are referenced, of course).

-- Copyright (c) 2012, 2016, 2018, 2021, The Trusted Domain Project. All rights reserved.


This is the Trusted Domain Project's impementation of the DMARC protocol libary and mail filter, called OpenDMARC. A "milter" connects to unix-based mailers (originally, sendmail, but now many) and provides a standard filtering API.