Cryptographically signing messages ensures recipients that the message really did come from you. For this to be meaningful, someone else must sign your key to verify that the key belongs to you, and that you are really you. That is why there are key-signing parties. See GnuPG Keysigning Party HOWTO for how to do this. Once it's set up you may send signed messages only, or signed and encrypted messages. Again use the Options menu, or look for the little quill pen icon.

When you are configuring Crypto Backends you may see the checkbox for S/MIME grayed-out. This occurs on Debian because S/MIME support is not compiled into GPGME, as of KMail 1.7. Why this is I do not know. You can still use encryption. When you send a message, choose "Inline OpenPGP (deprecated.)" Or you can build GPGME from sources and include support for S/MIME.

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