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DKIM and ARC features for Sympa

DKIM (RFC 6376) is a cryptographic signature method designed to prevent phishing. As postmaster or listmaster you should consider:

  1. checking the DKIM status of each incoming message
  2. signing all or a subset of outgoing messages

ARC (RFC 8617) is intended to fix the problems introduced by DMARC by adding signatures (they are called "seals") that show the chain of servers that processed a message. Once ARC is more widely implemented, workarounds by DMARC protection shouldn't be needed. So setting up ARC on your mailing list server is recommended, so that impact by DMARC protection would be reduced (Once you have DKIM set up, adding ARC seals is straightforward).

Processing of DKIM status of incoming messages is done by the MTA (Mail transfer agent) that delivers emails to the Sympa server Sympa domain (in order to reject some messages). This topic is not addressed in the chapter of the documentation.

You must consider signature of outgoing messages. Should messages broadcasted by Sympa to list subscribers be signed by your organization? Should all of them be signed? Should a subset of trusted messages be signed? Should service messages (automatic answer, welcome messages etc) be signed? In most cases Sympa should sign all the mail it sends to get the most benefit from DKIM.

ARC seals only make sense on mail forwarded by Sympa. The ARC feature will only add seals to those messages. ARC checks the ARC seals, if any, on incoming messages,


  • On Sympa prior to 6.2.72, ARC seals were added to only the messages sent through the list posting addresses. On Sympa 6.2.72 or later, ARC seals are also added to the messages forwarded through the addresses of listmaster, list owners and list moderators.

  • On Sympa 6.1 to 6.2.70, before enabling DKIM feature you may have to update your customized scenario to introduce dkim authentication method. See "The dkim authentication method for scenarios".


Prerequisites for DKIM

DKIM features in Sympa are based on the Mail::DKIM cpan module ; you should install it first. Check the documentation related to cpan modules installation.

To enable Sympa's DKIM signing feature, you have to publish public key for signature verification as DNS resource record. That is, you have to have the permission to add DNS RR.

Prerequisites for ARC

ARC requires Mail::DKIM version 0.57 or later which includes ARC support.

Like DKIM, ARC requires the keys for signing, but the DKIM key may be diverted for it.

It also requires that the front MTA that delivers emails to Sympa adds an Authentication-Results: header field that shows how a message was (or wasn't) authenticated as it arrived. You will know how to do it in succeeding chapters.

Configuration instruction

  1. Setup key pair for DKIM / ARC (for DKIM signing and/or ARC)

  2. Setup MTA (for ARC)

  3. Setup Sympa

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