Digital Sign and Shared Mailbox

Hi all,

Can anybody tell me what steps need to be done to let user sent digital sign message from shared mailbox?
He have his own certificate he can sent sing messages from his private mail but when try to send it form shared mailbox he get error "Invalid Certificate", dose the user need to get shared mailbox certificate or is there any other way to set up it? We work on Exchange 2010, Outlook 2010 and Windows XP.

Regards

Kamil