On the Windows PC/Server of the email recipient, the certificate must be installed.
To install certificate on Outlook:
1. Open Outlook. From the File tab, choose Options, then Trust Center, and then Trust Center Settings.
2. Click Email Security, and then Import/Export.
3. Click Browse.... Locate your certificate file (.p12) and click Open.
4. Enter the password used to secure the private key, and click OK.
