Hi. Is the server located locally? as in your computer? If so it may be trying to send mail using the localhost SMTP server and the default address (which is something like "admin@localhost"), if this is so then you might not have SMTP software installed. There is a way round this however, by making two calls to the "ini_set()" function:
1. ini_set("SMTP", "your ISP's SMTP server");
2. ini_set("sendmail_from", "your ISP email address");
This function returns the old value of success or false on failure.
However.... if the server isn't yours then your best contacting your server admin about it. Good luck.