“The e-mail address in the signing certificate [mail address] does not match the sender’s address”

I am running Outlook 2011 on my Mac. In Outlook I am working with certificates to sign my messages. If I send out emails, that works fine, but if I receive emails - no matter from whom - I receive the following error:

"The e-mail address in the signing certificate [mail address] does not match the sender's address."

I have checked the www for any help but have found none so far.

Does anyone have an idea what might cause it and how I can get rid of this message.

Thank you in advance.