Well - do you want to have a 'real mailserver' (which requires lots of work) or do you want to 'send mail'? They are two totally diiferent things. If what you want to do is 'sendmail' from a script or application (could be your own 'program') - then the tool is ~ TADA ~ SENDMAIL - not Mercury which is a full blown SMTP and POP mailserver.
If you need to hitch a PHP program to send mail by way of an e-mail account you may have, its the right tool. Its found in \xampp\sendmail There's a sendmail.ini file that needs configuration to point to the default account, and you will need to make changes to the MailFunction section of php.ini.
Here are two threads that discuss this - (including one about XAMPP lite)viewtopic.php?f=16&t=42756&p=168406&hilit=sendmail+JonB#p168406viewtopic.php?f=16&t=39649
(does the last one sound familar??? LOL)
IF you really want to run a mailserver (send/receive, and store e-mail for a domain (or localhost) - that is a totally different ball of wax.
Good Luck - and let me know if you actaully need to run a mailserver...