For making your PC a mail server that can be used to send emails by Internet, you should have the following - you pc must have a static dedicated IP address - Your PC IP address should have the reverse DNS entry If you have above things achieved, then you just need to install any mail server software such as hmail server, smartermail, etc.
You can have those 2 things configured by your internet service provider(Internet connection provder). Contact them for setting up these. You may need to pay the extra charge for dedicated IP address.
This is good for MX servers (incoming mail) but if you simply need to send mail, you don't need those features. Regards Thibaut
Have a look here: http://www.tech2all.com/2006/03/12/how-to-setup-your-own-e-mail-server/ Don't open your SMTP port to the Internet as you may get lots of attacks and spam attempts. (Check my sig) Also forward all of your emails to the SMTP of your ISP as many MX won't like your dynamic IP. Hope this helps. Regards Thibaut