DKIM on Postfix:

https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-dkim-with-postfix-on-debian-wheezy

The configuration can be tested by sending an empty email to check-auth@verifier.port25.com and a reply will be received. If everything works correctly you should see DKIM check: pass under Summary of Results.

==========================================================
Summary of Results
==========================================================
SPF check: pass
DomainKeys check: neutral
DKIM check: pass
Sender-ID check: pass
SpamAssassin check: ham
Alternatively, you can send a message to a Gmail address that you control, view the received email’s headers in your Gmail inbox, dkim=pass should be present in the Authentication-Results header field.

Authentication-Results: mx.google.com;
spf=pass (google.com: domain of contact@example.com designates — as permitted sender) smtp.mail=contact@example.com;
dkim=pass header.i=@example.com;