Setting up the danish public digital signature with Mail.app is not obvious, but this is what I did to make it work (needed it for setting up ePosthuset with my gmail account):
First install the Digital Signatur from DanID. You should end up with a .pkcs12 certificate file, probably located in ~/.oces Install the the certificate in your keychain. This is normally done by just double-clicking the .pkcs12 file. This should also open the Keychain Access application (Hovednøglering if you’re on a danish system). Locate your certificate—it will probably have your name, eg. Tobias Haagen Michaelsen for me.
Right-click the certificate and select “New Identity Preference…” (da: “Ny id-indstilling…”) and enter your e-mail address. Now comes the tricky part: As I have set up the account in Mail.app with my name and e-mail address, it doesn’t immediately recognize the identity because it has the e-mail address as the name as well, so you have to double-click the identity and change the name to match your Mail.app account.
And after restarting Mail.app it now Works on My Machine™
