Reply to signed E-Mail with Apple Mail

I suppose you refer to S/MIME encryption and signatures.

When you receive a signed email (you see the seal symbol below the header of the email), you should be able to simply reply to the email. On the right of the reply email, there should be a lock symbol now that should be in locked state. You can toggle the state by clicking on it.

If Mail does not allow you to toggle the symbol to the locked state, return to the received email and click on the seal symbol. The associated certificate should be displayed now with an option to import it into your keychain if hasn't already been imported. After that, try above steps once again. It may be necessary to restart Mail.