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by upofadown 1595 days ago
A sender can throw anything they want in the "From:" field and then sign it. The receiver does not have to agree. What would happen is that the receiver would see that the holder of the domain was different than the domain in the "From:" address and on the basis of bad "domain alignment" could reject the email.

I now think that the DMARC stuff is a red herring and would actually help make the current mailbox.org behaviour not all that problematic (they specify "reject" in their DMARC policy). The actual point of dispute is the lack of enforcement of the "From:" address domain.
2 comments
brewmarche 1595 days ago
Yes, lack of enforcement by mailbox.org on the Header-From when signing DKIM is the problem for DMARC IMO. It means I can’t trust a DMARC pass due to aligned DKIM.

Mailbox.org’s servers have access to 4 private keys as far as I know. These (I mean the matching public keys) are stated in mailbox.org’s DNS records. If you send from an @mailbox.org address you trust mailbox.org to do checking on the Header-From when signing it, as you have no control over which keys you state in DNS. This is the same situation as for any mail provider with a shared domain.

What’s even worse, when using mailbox.org with a custom domain they will have you state the exact same 4 keys in your domain’s DNS records for DKIM to work. There is no way to upload custom keys. So even someone with a custom domain has to trust mailbox.org to not sign strangers’ e-mails.

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upofadown 1595 days ago
The DKIM key is in your DNS. Does mailbox.org provide a DNS service and somehow enforce what you put in there?

Added: Wait, how would that even work? You need to generate your own DKIM key.

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brewmarche 1595 days ago
No one is forcing me, but not using it would mean I can’t have DKIM signatures, wouldn’t it? As far as I know there are no mail clients that add the signature before sending it to the MSA, but I might be wrong.
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brewmarche 1595 days ago
Re the addendum: No, mailbox.org does not support own keys.
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brewmarche 1595 days ago
But since DMARC will also give a passing result with aligned SPF, the Header-From checker has not only to refer from adding a DKIM signature but actually reject the e-mail completely for DMARC to be reliable.
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