On Wed, 6 Nov 2019 at 16:12, Roberto Carna <[email protected]> wrote: > My cooperative mail server is an Exchange which does not implement DKIM at > all. > But also I have a Postfix mail relay for the "example.com" domain. > Is it possible to implement DKIM only in my Postfix server for all the > outgoing @example.com mails ??? Or doing this I affect the outgoing mails > from my Exchange server because it sends @example.com mails withouth DKIM > mechanism ???
It is possible, but in my opinion pointless. In fact DKIM without DMARC is problematic at least, precisely because there are organisations which send some emails conforming to one or other standard (or both) and others emails which do not, and the recipient cannot be confident that non-conformant emails should be rejected - the very situation you have in mind. In theory adding DKIM to some emails should not cause any problems. Might some recipient MTAs see that your domain has a DKIM record in DNS and then 'downgrade' (treat as spam) or block emails from such domain that don't have DKIM? I am not sure how big this risk is, but I can't see you gain anything by running it. Are you sure your Exchange server can't implement DKIM?
