Mail on OS X: Choose which X.509 certificate to use for signing mails
You can exactly specify the certificate to be used for sining outgoing emails by following these steps:

Open your keychain. Then right click on the list of certificates and pick the first item on the context menu (In English it is labeled New Identity Preference):



Then type in your email address and select the certificate from the drop down box to use for that address:



The next email you send from the email address typed in above will be signed with the selected certificate.