Apple Mail - Email Certificate: Untrusted Signature

When I send emails to other people, they are getting a notice that my S/MIME certificate is an "Untrusted Signature." I've already gone round and round on this. My hosting provider says it's not them, not their problem, "you have to talk to Apple."

I found what looked like a solution to a similar problem here on the Apple Discussions Board, and followed the instructions, and it did not fix the problem. The problem persists. I HAVE TO FIX THIS. I am a freelancer, and I use this email address for professional communications.