Importing the SMIME certificate into Outlook
Make sure you have imported a valid certificate onto the computer you are using (link to the FAQ articles: Request an S/MIME certificate).
In Outlook, go to the File and select Options and, in the Security Centre select the Security Centre settings :
Under the ‘Email Security’ sub-section, you can first enable the setting ‘Add digital signature to outgoing messages’. This setting is recommended, as it ensures that email messages are signed by default. Also enable ‘Send signed messages in plain text’.
Then click ‘Import’.
In the window that opens, enter the path to your backup copy of the certificate, the password you used to protect your backup copy, and your name. Click ‘OK’.
In the following dialogue box, please select the ‘Security level’ option
By default, the ‘Medium’ option is selected.
We recommend the ‘High’ option. This means that a security password will be requested when accessing the certificate’s private key, which is required for signing. You must set this in the next dialogue box.
When using the application, the private key is stored in Outlook’s memory for a while; you do not need to re-enter it for every email.
You now need to adjust the settings in the main ‘Email Security’ window:
Select the signature certificate. The certificate you have just imported will be offered to you:
Note: If you have installed several S/MIME certificates, you may only be able to select the current certificate after clicking the ‘Further options’ link; you can then select the current certificate.
Example: 
After selecting the certificate, choose the following hash algorithm: SHA256 as the hash algorithm:
When composing a message or email, you will now find the option in the menu bar under Options Encrypt and Sign. Signing is enabled by default.
You can only encrypt messages if you have previously generated the public key . This requires that the recipient also has a valid S/MIME certificate and that you have received this, for example, as an unencrypted signed message.
English Version: Import certificates into Outlook (Windows)