From Half-Eaten Software:
POP3Server OCX is an ActiveX control for Visual Basic or Visual C. It is a stable, fully-functional POP3 e-mail server that can be incorporated into your own programs. It allows POP3 e-mail clients such as Outlook Express or Mozilla Thunderbird to connect to it and download a user's messages. New mail can be composed from directly within your program using a simple command. It could also be used in conjunction with a SMTP server to create a complete internet e-mail server.