Not a trusted certificate. ??

I have one of these digital signatures. It seemed to be working well and I really do appreciate the free service.

However, today when I attempted to e-Mail Techsmith’s tech support at custservice@techsmith.com, I received a message back that the message was rejected because the certificate was not a trusted certificate.

The exact wording was:

There was a problem with your S/MIME signature certificate:
The certificate used to sign your email is untrusted.

I was advise to correct this and re-send the message.

Of course, my solution was to just turn of digital signatures for that message so that there was nothing to check (and return) and then re-send the message which was accepted with no problem. I guess I understand that if a customer elects to use a digital signature, then Techsmith feels obliged (or their server is set to) check that certificate.

What confused me was this reject message.

Ideas please??