Verschlüsselung über Thunderbird

Other people's certificates  
To send encrypted messages to other people, you must have their SMIME encryption certificate (.cer) in the "Other People's" tab of your Certificate Manager. Thunderbird automatically adds other people's SMIME certificates to that tab when you receive form them a digitally signed message with a valid signature and with an SMIME certificate issued by a recognized and trusted Certificate Authority (CA). CA certificates that appear in ThunderBird's "Authorities" tab are recognized, and may also be trusted. CA certificates that do not appear in that tab are considered "unrecognized".  

An SMIME certificate that was issued by an unrecognized CA will not be automatically added to the "Other People's" tab of your Certificate Manager. If you attempt to manually import an SMIME certificate that was issued by an unrecognized CA, nothing will happen--literally. Thunderbird will not even display an error dialog. It will just not import the SMIME certificate. This is generally not a problem when receiving an SMIME certificate that was issued by a trusted Certificate Authority (CA) such as Thawte and Verisign, but could be a problem for a certificate that was issued by an unrecognized or untrusted CA, or for a certificate that is self-signed (i.e. it has no CA other than itself).  

So, before you can import an SMIME certificate that is issued by an unrecognized CA or is self-signed, you must first acquire and import the certificate for the issuing CA. In the case of a self-signed certificate, you will need to acquire a ".cer" file from the individual whose certificate you wish to add.