You can use kmail and gpg to send encrypted or digitally signed mail. To do this, there are the buttons "sign" and "encrypt" in the kmail compose window. It appears if you click on "File|New|New message".

TroubleShooting

Whatever problems arise, first make sure your gpg is properly set up:

gpg --list-keys

Has to deliver something like

/root/.gnupg/pubring.gpg
------------------------
pub   1024R/307E3D54 2006-03-21 [expires: 2010-05-05] 
uid                  SuSE Package Signing Key <build@suse.de>

pub   1024D/9C800ACA 2000-10-19 [expires: 2010-05-05]
uid                  SuSE Package Signing Key <build@suse.de>

and much more lines.

Check that

gpg --update-trustdb

does not deliver

gpg: no ultimately trusted keys found

If you have a keyring saved to your disk, reload ("import") it like this:

cat secring.skr | gpg --import

Then call kgpg and set your trust on this key to ultimate.