I've had no issues making DKIM work with local domains. However, now I'm at the point where I'm trying to implement DKIM on a domain I don't actually host, and one for which I'm being asked to send email from.
First, I set up the domain as a smart host, with the appropriate forwarder to the real mail server handling that host. I've tested this and it works fine. The people in that domain can receive my email.
Second, I turned on DKIM for the smart hosted domain.
Third, I sent the DKIM string to the DNS administrator of the site, who has correctly put it into his server.
Fourth, I created an email account in the postoffice that include the domain that's acting as a smart host.
When I send email using the email account to gmail for testing, it's coming up without the "signed by" expected.
I'm running version 5 professional.