Free / Opensoucre mail server for xp

[-BobW-]

Windows 2000/XP workstation works fine as a web server/email server/database server.  

Microsoft doesn't like it much but there really isn't much difference between Windows 2000 server and Windows 2000/XP professional when it comes to running stuff like this.  They want you to pay for a server license anytime you run server software.

I've run Mysql, IIs, Apache, MSDE (MSSQL Developer edition) and others with no issues in a production environment.

[-BobW-]

@Piru

"Only single user can use the XP machine at a time. Thus I consider it singleuser."

That really doesn't matter when using XP as a server.  Very often you don't need interact with the desktop and when you do it's pretty unlikely that more than one person needs to do it at a time.

1 - Much of the server software can be managed remotely.  You can start and stop services, copy/edit files etc.  10 people can do this at a time easly.

2 - Things like databases have client tools to do admin and database management.

3 - If I remember correctly there are telnet servers available for Windows workstation.