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Mailman's transformations can break DKIM signatures. The official DKIM specifications about best common practices in the face of message transforming mailing list remailers is still in draft form - dkim deployment RFC. Of larger importance than DKIM signatures is the effect of RFC5617 Author Domain Signing Practiceswhich defines which domains are DKIM signing all email. There is a draft spec being proposed that directly addresses issues involving DKIM and MLMs.
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