[Users] can't reply to e-mails with padlock icon displayed at bottom

[Gil Weber]

I am trying to correspond with someone sending to me from a military
e-mail address. The e-mails come to me in Claws-Mail digitally
signed and displaying a padlock icon at the bottom of the screen
and a message that says "The signature has not been checked. Click the
icon or hit 'C' to check it."  

I do that and an "OK" appears over the padlock. The message next
to the padlock changes to "Good signature from 'name of the person who
sent message and a number."

So I assumed that OK means I can reply.

However, when I try to reply a box pops up in the middle of the screen
saying "Error S/MIME Cannot Sign: General error (1)"

It does not matter if I remove the military e-mail address and insert
this person's private e-mail address into the address bar. The same
thing happens if I try to reply to these messages.

I am informed that the e-mails are signed with a CAC certificate (I
have no idea what that means).

Is there something in the preferences tab that I need to click to allow
me to reply to these digitally signed e-mails? Am I missing something
obvious?

Thanks!
Gil