Trust S/MIME Signing Certificates in Outlook

By default Hermes SEG uses self-signed S/MIME certificates for encryption and signing. E-mail clients such as Outlook do not trust those self-signed certificates. In Outlook, emails signed with the self-signed certificates show an invalid digital signature error (Figure 1).
Figure 1
In order to have Outlook trust the certificates, click on the yellow triangle to the right of the e-mail in question (Figure 2).
Figure 2
In the resultant Digital Signature pop-up window, click on the Details button (Figure 3).
Figure 3
In the resultant Message Security Properties pop-up window, click on the Signer e-mail address and then click on the Edit Trust button (Figure 4).
Figure 4
In the View Certificate pop-up window, in the Trust tab, under the Edit Trust section, select Explicitly Trust this Certificate and click OK (Figure 5).
Figure 5
Close and re-open the e-mail in question and the yellow triangle is now replaced with a red ribbon indicating that the Digital Signature is trusted (Figure 6).
Figure 6