How to send email from localhost to Yahoo.com via Mercury?

[yosimba2000]

I have Mercury service started, but my PHP is returning error: Warning: mail(): Bad Message Return Path

I've followed some guides and made some changes to php.ini and sendmail.ini and restarted Mercury, but it did not resolve. I've since reverted all changes so it should be back to stock. Can someone walk me through setting this up, because I really have no idea what I'm doing.