Galaxy S3/Android: "Unable to verify signed message" and inability to reply.

I have my Exchange account set up on my Galaxy S III. The phone is great, but I'm coming from several years of iPhones, so it's taking me a little while to get used to it.

I received an email from a person today. In the message list, there is a little gold and red icon (that looks like of like a seal or medal). I can open and read the email fine. At the very bottom of the message, there is a button labelled "Verify Signature." When I click it, I get an error message that says, "Unable to verify signed message. Certificate not installed."

Furthermore, if I attempt to reply to the email, I get two messages that flash up (briefly) at the bottom of the screen:

1. "Unable to sign or encrypt message. Certificate not installed."
2. "Quoted text not available. Replying to unicoded S/MIME message."

I can type a reply, and I can save it as a draft, but if I attempt to actually send the message, I get that first "Unable to sign or encrypt message" error again.

I can reply to his email fine through Outlook or OS X Mail. Is there no way for me to reply to these types of messages on my phone, though? Thanks.