I always change the default email account that Wordpress uses to send mail to something unique because of this very reason. There is no way you can prevent scammers from imitating an email address.
There are several plugins that will change the Wordpress From Email info, but to do this programmatically you can use:
function hidden_mail_from($old) {
return 'wordpress_secure@yourdomain.com';
}
add_filter('wp_mail_from', 'hidden_mail_from');
function hidden_mail_from_name($old) {
return 'My Wordpress Install 1234';
}
add_filter('wp_mail_from_name', 'hidden_mail_from_name');
If you aren't a developer I would recommend the SMTP plugin: http://wordpress.org/plugins/smtp/
And most of the other SMTP plugins on the Plugin Repository: http://wordpress.org/plugins/search.php?q=SMTP
If you are really interested you can look at the mail headers in your email client to determine the origin server of the email and potentially create a rule to filter spam out based solely on that header but the plugins are much easier.