SPF/DKIM/DMARC and mailing lists / 2022-03-26

2022-03-26 SPF/DKIM/DMARC and mailing lists
One of the founding forms of information exchange and community building on the Internet is the mailing list. A subscriber sends mail to a central mail address and the mail gets redistributed to all members.

As this mechanism has been abused by spammers in lots of ways there has been a lot of work in stopping unwanted mail being distributed by mailing lists.

There has also been a lot of work in publishing the official way in which outgoing mail from organizations is handled: Sender Policy Framework (SPF), documenting the sources from which e-mail can be send, DomainKeys Identified Mail (DKIM) for signing outgoing mail headers and body and Domain-based Message Authentication, Reporting and Conformance (DMARC) for publishing the policies for mails that fail SPF/DKIM and reporting on those. The way mailing lists forward mail isn't really compatible with SPF and DKIM. There is a 'new' source of mail from the original sender and some headers are changed/added when forwarding it with mailing list software.

Yesterday I sent something to a mailing list from an idefix.net address and this morning I see a number of dmarc reports with failures, because the mailing list server isn't authorised to send on behalf of idefix.net. So maybe some people on this mailing list haven't received my reply. In the long run lots of SPF errors from this IP could also hurt its 'reputation score' for outgoing e-mail. Some mailing lists 'fix' this by not allowing domains with strict spf/dmarc policies, others go through interesting adjustments with 'sent on behalf of'.

I have no simple solution for this, I see an example of security measures breaking an existing use case, for which adjustments may have to be made.

Update: The general approach here seems to be 'sender rewriting'. Recently updated mailing list software should support this. But it depends on the mailing list owner to check the settings and update the software.

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