[ Thanks to Falko
Timme for this link. ]
“S/MIME does not work in the direction you might think it does.
You cannot send encrypted mails once you have got a valid and
trusted certificate yourself – once you have that, you can receive
encrypted mails if it was encrypted with your public key.“S/MIME works as a paired-key-system with a public and a private
key. Your public key has to be given to the ones that want to send
encrypted mails to you, while you keep your private key that is
able to decrypt those incoming mails. That means, if you want to
send encrypted mails, the receiver has to use S/MIME as well and
you need to know his or her public key to encrypt messages. To give
someone your public key, it is only necessary to send him or her an
email signed with your certificate.“I perform this tutorial on a machine with Linux Mint 11
installed, so all commands that apply to that have to be altered
depending on your distribution.“The steps for importing certificates into Thunderbird are the
same on Linux as they are on every other operating system. This
means, these steps can equally be used on Windows or Mac.”