Mailservers can only work with REAL DNS entries (since you have named a host presumably).
"localhoster.org" is presumably a fictitious domain. So unless you actually have registered 'localhoster.org', and have control of its Nameservers, you won't be able to 'point to' its IP for where to pick up and send mail - that's what a mailserver has to do.
By adding an entry to your Hosts file, you can supply the hostname and IP
The first presumably would be the first of the 'Mx' records in 'real domain' localhoster.org, if such a thing existed.
Mailservers work much like webservers in this sense, you can configure them to send and receive mail from multiple domains on the same IP, so effectively -- each mailserver instance (host) works just like a NameVirtualHost vhost setup, it has to discriminate WHICH domain you want to configure/talk to/query etc.
IF you only wish to "send mail" from a web application (such as a PHP script, a blog, forum or what have you, or just to use PHP or Perl mail functions - you ONLY need Sendmail. see this - (I have a remote down router I need to fix -- I'll post a complete link later) There's a fairly complete 'how-to' for XAMPP sendmail for Windows.
this is the Google cached copy - http://webcache.googleusercontent.com/s ... us&strip=1