Signing let's you prove what you did write, it doesn't let you prove what you did not write, since all the signed emails in the world don't prove the nonexistence of unsigned emails from the same sender. Now, if you have a regular practice of signing emails, one without a valid signature that purports to be from you is pretty strong evidence that either you didn't send it or you anticipated the need for future repudiation, but it doesn't itself prove anything about which of those applies.