1. Set up digital ID

After starting Outlook, open the menu „Datei“ (“File”):

There you open the „Optionen“ resp. (“Options”):

In the “Outlook Options” you select the “Trust Center” page and open the „Einstellungen für das Trust Center“ (“Trust Center Settings”):

In the “Trust Center“ you select the page „E-Mail-Sicherheit“ (“Email security”), activate the setting „Ausgehenden Nachrichten digitale Signatur hinzufögen“ (“Add digital signature to outgoing messages”), and open the dialog „Importieren/Exportieren“ (“Import/Export”):

There you select the PKCS#12 file containing your digital ID (i.e. your private key, your public key and all relevant certificates) (not the PEM file from the email):

Furtheron, you enter the passphrase (password) protecting the PKCS#12 file and start the import process with “OK”:

A dialog box appears. Here you click on „Sicherheitsstufe“ (“Security level”):

Select the security level „Hoch“ (“High”; this causes Outlook to ask for your passphrase each time when accessing your private key, i.e. each time you are signing or decrypting) and click on „Weiter“ (“Next”)

(If your PC is secured by IT experts, if your PC is used only by you, if you always lock your PC when you leave it, and if the PC is located in a locked room to which only a few trustworthy people have access, then you are also welcome to select the „Mittel“ (“Medium”) security level and skip the next image in these instructions. This will save you from having to enter the password every time you sign and decrypt emails).

Now you are asked to choose a password. This password will be asked for before using your private key for signing or decrypting:

You get back to the previous dialog box. There you check that you have really selected the high security level and complete the import process with “OK”:

Back on the “Email security” page of the “Trust Center”, check that signed messages continue to be send as clear text, and open the „Einstellungen“ (“Settings”):

Her you check that the correct certificate is selected both as „Signaturzertifikat“ (“Certificate for signing”) and as „Verschlüsselungszertifikat“ (“Certificate for encrypting”).

Addendum (September 2023): If you are not importing a digital ID for the first time: With new Outlook or Windows versions, please be sure to click „Weitere Optionen“ (“More options”)" in the „Wählen Sie ein Zertifikat aus“ (“Select a certificate”) dialog box, because only then will you be offered all valid imported digital IDs for selection.

Select the „Hashalgorithmus“ (“hash algorithm”) “SHA256” and the „Verschlüsselungsalgorithmus“ (“Encryption algorithm”) “AES (256-bit)”, and confirm the settings with “OK”:

(If there is no „Name der Sicherheitseinstellung“ (“security setting name”) entered at the top, simply select a name yourself.)

These settings are both very secure and compatible with most other software:

Close the “Trust Center” with “OK”:

Close the „Outlook-Optionen“ (“Outlook options”) with “OK”, too:

This completes your settings.

2. Try out digital ID

Now you can try that sending signed or even signed and encrypted emails really works. Hence start a „Neue E-Mail“ (“New email”):

In the toolbar at the top you can see that signing is activated by default and that both signing and encrypting can be activated or deactivated with a single click.

Leave the default settings unchanged and start a new email by selecting a recipient. You should be familiar with these steps:

When ready click on „Senden“ (“Send”):

Because you want to sign the email and need the private key, you are now asked for the password you have established when importing. Please enter it and allow using the key:

The email is now sent. You find it in your “Sent” folder. Look at it and see the symbols indicating a good (sucessfully verified) signature.

You can click on the symbols to get further information (not displayed here).

Now write a new email. This time it will be encrypted, too:

The steps are the same as when sending a signed email, but this time we activate encrypting:

If the certificate of the recipient is known to your Outlook, you hardly see a difference. (Otherwise ask the recipient to send you a signed email. From that email, you can import the certificate as described below.)

If you look at and open the encrypted email, you see the corresponding symbols. If is has not already occurred, you are asked for the password of your private key.

3. Import the sender's certificate from a signed email

In contrast to most other email programs Microsoft Outlook can send encrypted emails only to addressees saved with certificate in an addressbook.

Hence you should add senders of signed emails to your contacts to allow for later sending of encrypted emails to them.

So if you read a signed email, click with the other (right) mouse key on the sender:

From the context menu select the point „Zu Outlook-Kontakten hinzufügen“ (“Add to Outlook contacts”):

(If this menu entry is not displayed, the sender is already saved as contact.)

Save the new contact:

And close the dialog window:

From now on, you can send encrypted emails to this contact. To do so, do not type the email address but select the recipient from your address book, as shown above.