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So, at the moment I am signing mails from specific domains using opendkim and this works. But when I try to send mails from domains that opendkim does not know, they they are not signed.

What I want to do, is to make opendkim sign all emails going through postfix using a single domain, just like mailchimp etc is doing.

Example: I am sending emails from myname@mydomain.com using mailchimp, and the signeture is something like the following:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; s=k1; 
d=mail2.suw13.rsgsv.net; h=Subject:From:Reply-To:To:Date:Message-ID:List-
ID:List-Unsubscribe:Sender: Content-Type:MIME-Version; 
i=*****mydomain.com@mail2.suw13.rsgsv.net; 
bh=4Rla76/wHV31ER3IZqXOuA09j3OG2SuFbfD5Jc7Kn94=; b=17jmyvz05JfeNC+avqWJmtESF2A58LA/CievFVtQ0sqwo4FYKAP0Gfpjtc5LSG7tr9ntS5CziAgSOa+UyEjRP3AhZOOXDFoQMUG0gntqxg/gP074Vi7Hy0XvFzAiJYZfAhijwvaroY45hjuX+RN3nQ0xTfhWem5mv3+VVYpwvUo=

How do I achieve this?

  • You cannot achieve this at all, as DKIM has two parts to configure. opendkim is one side, the other side is to put the public keys for your domain into a DNS record. So a "catch all" configuration cannot work as opendkim does not which key to use for which domain or even if one of its keys is correct for the domain. – allo Jan 29 '18 at 15:49
  • If you want to use some automation, you can try a tool like ansible to generate opendkim configurations and DNS zonefiles for your domains. – allo Jan 29 '18 at 15:50
  • I see your point. But what am I missing? I received an email from and would like to do the same. Example: Company Support <support@company.com> via mail136-21.atl41.mandrillapp.com, and later: signed by: mail136-21.atl41.mandrillapp.com How do they achieve this? – BQffen Feb 27 '18 at 20:09
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like this answer the trick lies in the use of SigningTable and KeyTable:

/etc/opendkim.conf

...
SigningTable    refile:/etc/mail/dkim_signing_table
KeyTable        csl:keyname=example.com:selector:/etc/mail/selector.key 

/etc/mail/dkim_signing_table

* keyname

So the SigningTable maps all domains to a key, and the KeyTable provides a domain/selector for the keyname.

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In your example, the message isn't signed for your own domain, but for d=mail2.suw13.rsgsv.net, instead. MailChimp has that as their default authentication, but it recommends using Custom Domain Authentication i.e. having an own DKIM authentication for every domain.

It removes the default MailChimp authentication information ( "via mcsv.net" or "on behalf of mcsv.net") that shows up next to your campaign's From name in certain email clients.

For the same reason you shouldn't try to have a single domain for signing messages for all domains.

  • according the the sprit of the RFCs you should even try to sign each type of message in a domain with different signing domain. – danblack Aug 19 '18 at 11:48

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