This is a good application for use on my combination Windows and Macintosh network which is "secure" in that it does not involve the internet. Email messages and files are sent directly from one PC to a Mac or another PC on this and only this local area network (LAN). This prevents information from leaking out onto the internet.

I have conducted a search, and have not bound any other LAN only cross platform email program that does not require a server, and is secure from outsiders.