Sign mails using Thunderbird or SeaMonkey
Note: Some small differences might appear between versions of ThunderBird and Seamonkeys.
Run ThunderBird or SeaMonkey
Menu Mail and Newsgroup account settings
Select Security
In Digital Signing click Select
Select the User Certificate you want to use to sign your outgoing emails and click Ok
Select No in the following warning popup
Check Digitally Sign messages (by default)
Click OK