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I installed DKIM Verifier plug-in in my Thunderbird mail client, just to see how this DKIM technology works. I don't really understand details of that technology, but it seems to check the signature of the sending server.

I have noticed that much of the spam I receive has a DKIM signature, so that has me worried; Can spammers see when my mail client is checking the DKIM signature?

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  • So there is no direct communication from mail client to spammer controlled servers. There is just query thorough chain of DNS servers, and this query can also be cashed at various points. Because of this, it is practically impossible for spammer to get any feedback, even if he is owner of foreign DNS server.
    – Andrei
    Commented Jun 9, 2016 at 6:40

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The DKIM check is done by the server that is processing your mail. And your mail client does communicate with your mail server (your mail provider). That tells you that the sending email server is correct and the mail was not altered on the transport. Since many servers that are used for spam-mails have a correct signature (or are abused to send those mails) the system does not prevent all spam mail. But the check is not done over the sending email server (ask the sender of the package if he is a nice guy is not very helpful...) but over the domain name system.

-> sending email has one part of a key the domain name server the second (public)

-> your email server checks on the domain name server if the key of the sending mail matches

-> no communication to the original sender but with the DNS-Server

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  • So, my mail client is not making any direct communications to spammer mail server or his DNS server. DKIM checking happens only thorough my mail server and its DNS server?
    – Andrei
    Commented Jun 8, 2016 at 15:25
  • Thats how i read the standard some time ago,yes.
    – Matte
    Commented Jun 8, 2016 at 18:06
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As also stated in the Privacy Policy of the add-on, the only outgoing connections of the add-on are to the configured DNS server. So the only ones who could learn something form it, are the DNS server, an an attacker who listens to the un-encrypted DNS query.

The sender of the e-mail, i.e. the spammer, is not getting any feedback.

You can reduce the number of DNS query done by the add-on by enabling the caching of the DKIM keys.

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