Keep your privte key safe
You need your private key to read encrypted e-mails you have received or that you have sent. This key is stored in the certificate store of the computer or the mail program. If your private key is lost, you will not be able to read these emails. You should therefore store a password-protected backup copy of the key in a file, either in a filestore that is backed up or on an external storage medium, such as an external hard disk or USB stick, which you keep safe.
Save your key to a file from Outlook
To save your key to a file from Outlook, go to the "Trust Center"  in "Outlook Options", click "Trust Center settings..." at the right  and select "E-Mail-Security"  in the next window.
Here you will find a button "Import/Export..."  in the "Digital IDs (Certificates)" section. Activate the button "Export your Digital ID to a file"  in the lower area, select your DFN certificate, select an appropriate location and set a password to protect your private key in this file. This password should be of high quality and you must remember it well in case you have to use this backup of your key one day.
Save your key to a file from Thunderbird
To save your key to a file from Thunderbird, go to the "Tools" menu, select "Options" and select the "Certificates" tab under "Advanced". There you click the button "Manage Certificates". In the new window you will find your personal certificate in the tab "Your Certificates" in the section "Verein zur Foerderung eines Deutschen Forschungsnetzes", mark it and click the button "Backup...". Select a suitable location in the file system and protect the file containing your private key with a secure password. You will need to keep this in mind in case you have to use this backup of your key one day.
This process is also suitable for transferring your certificate with the keys to another computer and importing it there.