S/MIME functions on a system of a central authority that maintains the validity of the keys. eM Client will check with the authority if the key has been revoked pretty much every time the key is used. If the key has been revoked, or it can’t be verified, you will not be able to use it.
It’s done through the system API, so there may be some issue with your OS, or access to the S/MIME authority is being blocked on your connection.