From Deep Sky Technologies:
SMTP Client Deux is a 4th Dimension component which provides a complete implementation of the SMTP protocol. SMTP Client Deux works on top of the commercial TCP Deux 4D component. With the SMTP Client Deux component, a 4th Dimension developer has a complete implementation of the SMTP protocol from within 4D. The protocol relies on the TCP Deux component for all TCP communication needs, thereby removing any reliance on current or future TCP plugins which are available for use in 4D. The implementation of SMTP within the SMTP Client Deux component also eliminates long standing bugs and anomalies that have existed in plugin implementations of the SMTP protocol.