"Error: It is possible that the content of the message is changed"
We never had this issue before we changed our spamfilter. When this customer sends an e-mail to our Gmail address there is nothing wrong with the certificate.
My idea is that the spamfilter is adding a signature in their message (I already emptied the message, however, I cannot remove the last space) because of that the message has been changed. Therefore it seems a legit error.
You can clean all mail signatures in Anti Spam -> Email Signature