Lotus Notes

To use your certificate with Lotus Notes, you'll have to download it as a PKCS 12 file first.
In Lotus Notes:

• Open File / Security / User Security
• Select "Your Identity" then "Your Certificates"
• Select "Your Internet Certificates" (not "Your Notes Certificates")
• Select "Get Certificates" then "Import Internet Certificates"
• Provide the path to your PKCS#12 file
• Enter its password
• Your certificate's details are displayed. Click "Accept All"

When you receive a signed message open it and open Tools / Add Sender to Address Book". In the "Advanced" tab make sure the Include X.509 certificates when encountered" box is ticked.

To see the certificates linked to your contacts:

• Open "File" / "Security" / "User Security"
• Then "Identity of others" / "Show all in your address book"

See the official documentation