Email Encryption

I’m looking for some options for email encryption. I have looked at what Verisign has to offer but am not seeing anything that really fits my situation. With security more of a concern than ever, we want to be able to encrypt email and attachmentsthat we are sending to clients. So far everything I’ve seen requires both the sender and the receiver to purchase digital ID’s. Our receiver’s are not going to want to purchase an ID every year to recieve 1 or 2 emails as we are not going to want to purchase an ID for every one of our clients. Any suggestions?