Getting KMail to let me encrypt to an unsigned key?

[Adam Funk]

(Two apologies: this is slightly off-topic, and I've also posted the
same question to the debian-user list.)

I'm running the Debian kmail 3.3.2-3 package and gpg 1.4.3 compiled
from the source.

As far as I can tell, it flatly refuses to let me encrypt a message to
any key that doesn't have a signature chain back to a trusted key.  I
can see the usefulness of a warning about doing this, but I've
accidentally sent a message unencrypted while trying to find a way
around the problem.

Is there any way to override this restriction?