I followed these instructions to setup OpenDKIM with Postfix, and it works as advertised. My outgoing mails have the DKIM-Signature header added, for example:
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=example.com; s=mail; t=1398638300; bh=mk/7yYUxFCWz+ZHB0opJIA/S3J5ELoPZPfAO0KQdVg4=; h=Date:From:To:Subject:From; b=fPPfUliJUgA0re38nkJ2R18TeFgbamOv1U8nDb9958eTeAT6Mp7oq4WGrHPiPmc+b mrLu9RuW0/S4d0ipkilNZDxgecwl7qttrDbTEkWxdhwwTSe5FL3OBaUoUxJFrMGjmY RdBjY5ZWtvk29+gXZ+af5Of9OrY7COLlqGkFXRXw=
My question is about the
d= parameter above. In cases where one mailserver handles the outgoing mail of multiple virtual domains, should the d= contain the mailserver sending the message? Or should it be the domain that appears in the From: address?
I have tried to read RFC 6376 about this subject, but the only relevant bit I found was:
d= The SDID claiming responsibility for an introduction of a message into the mail stream [...] The conventions and semantics used by a Signer to create and use a specific SDID are outside the scope of this specification
I'm hoping someone here knows how the d= parameter is used in practice.