No doubt about seems to be an unique protty. Question is are you planning on implementing this by seat or server? If you force people to purchase the server software and system....are you going to provide unlimited client licensing? Reason being, this would seem to be a rather costly idea for security if corps had to pay for each individual system that needs to sendmail. Also, where is the redundancy? Most exchange servers have the option in case the main server is bogged down or taken down for other reasons. Will your software support the idea of load balancing or backup? Just some ideas.