To open encrypted emails, such as credit card details from your eommerce/shopping cart website, you need to install the private key corresponding to the public key used by your website to encrypt the details.

These are the screens in Microsoft Outlook Express. Other email programs have similar procedures.

Click →Tools → Options.
Click → Security → Digital IDs.
Click → Personal → Import.
Click Next.
Browse for the private key sent to you by us. It ends with .pfx
Enter the password that we sent to you for your private key.   You will only need this again if your computer crashes and the key has to be installed again.   Tick the boxes for security.
Place certificate in Personal Store.
Finish Importing the Key.
Set security level (Medium is OK).
Next
Create a password for daily use.   This will be required before opening any encrypted emails of credit card details, but only each time you open Outlook Express.
Click Finish — Cheers — you’ve done it!   You can now open encrypted emails.