This post is about the usage of the .NET SmartCard in Thunderbird. You might want to read Installing and configuring a .NET Smartcard first.
Go to Tools –> Options
Press Security Devices –> Load
Browse for the PKCS#11 DLL file which is located at the following location on my PC
C:\Program Files (x86)\Gemalto\PKCS11 for .NET V2 smart cards\gtop11dotnet.dll
Go back to the options-menu, now press “View Certificates”. Thunderbird now asks you for the SmartCard PIN and then shows the certificated contained on your card.
That’s it! You can now use Thunderbird with S/MIME. When you write an email, just select Options –> Digitally Sign This Message”.
Once the receiver got your signed mail, he can reply with an encrypted mail.