Before you find where to find cheap software for your mail server, I suggest you should understand first how you do it. Now, If you wish to make your very own email server to send out mass email, the main thing you have to think of is not a lot the box itself it's going to be sitting on, however the stuff around it. The registrations, reverse DNS and so on.
Initially, the first thing you require is to get a static IP from your service supplier. You can contact your ISP and ask them for a static IP, if you don't have it already. The second thing you have to worry about is reverse DNS lookup for your IP address, so when you do a reverse DNS lookup for your IP, you should get back your business domain.
Moreover, you could do for your very own server is install what is called SPF in your DNS. SPF is generally a string in your DNS settings. And that string shows a list of all the servers that are allowed to send email on your behalf for this domain.
Some suppliers, when they get email from you, would inspect if it exists. If it's not present, they may put your e-mail in the spam folder or decline it entirely. Or provide you less points, so to speak, in the spam check. An additional thing that's optional that you can fret about is DKIM- Domain Key Identified Mail- and you can get that if you want but it's really not necessary.
As for what software you can use, if you have a Windows box, you can utilize "Best SMTP Server" you can look it up on Google. Or if you have a Linux box, you can choose any SMTP server that's already there, like Exim or Sendmail.
I wish that answers your problem. Good luck