On 4/20/19 8:08 AM, Reto wrote: > On Sat, Apr 20, 2019 at 07:31:06AM -0400, Richard Damon wrote: >> Where the issue comes is with DMARC, which restricts the DKIM protocol >> to be aligned with the From line of the message, and thus the MLM can't >> make the message pass the DMARC settings of the sending domain. It is >> DMARC which breaks the traditional operation of a MLM, and the use of >> which implies that the sender should not be using such tools. > Now that's a bit dramatic isn't it? > A mailing list *can* work with dmarc just fine if it doesn't modify the > protected headers. > That doesn't seem to be particularely complicated assuming Headers like > List-Unsubscribe et al can still be added. > > Just don't modify the subject and the body and you should be fine. > One issue is that some legal advisors will state that List-Unsubscribe might not be good enough to meet the requirements of clearly visible unsubscription instructions since it is NOT required that a MUA handle that header in any particular manner if at all. That means that to clearly meet the legal requirement in those those jurisdiction must break the DKIM signature.
Mailing list also have traditionally modified other headers, like Reply-To to make replies by default to go to the list, and frequently the subject to allow for easy filtering of the messages from the list on MUAs that may be somewhat limited in their filtering capability. This was common existing practice, which DKIM acknowledged and provided for by allowing resigning aligned to the Sender, but DMARC broke. To meet the Mail RFCs the Mailing list should modify the Sender: field, so if that was signed (as was pointed out is recommended by DKIM) the signatures will be broken, and since DMARC requires alignment to From: (which the RFCs says should be the Author of the message, so should be the original sender), a MLM manager can be forced to break some RFC to be able to deliver the message. DMARC by itself isn't that bad, as the original intent was, at least in part, to protect transactional emails, and those should generally not go through the sort of system that have issues with it. The issue with DMARC was its adoption by mailing services that did not meet that model, and who did not inform their users of the implied restrictions on their users of the use of those services (and some [yahoo] even ran their own competing MLM service that for their users could 'cheat' and sign the modified messages). When the issue first came up, there was a move to ask the operators of mailing lists to just tell their subscribers from the domains that adopted the abusive settings that caused the issues that they were no longer allowed to post through the list (since that IS the real meaning of a strict DMARC setting), but it was decided that such a move would just be a punishment on the innocent and less technically competent user, and not those that ran the domains. There are still people who propose that to demonstrate the issue, that all MLM should be setup to automatically reject any message, and not use a RFC-incompliant workaround (like munging From:) that would create a DMARC exception, and some go as far as suggest that the rejection message should also be copied to the DMARC team to show them the extent of their breakage of the Mail System. -- Richard Damon
