No errors in logs at all ?
Is it possible that system is not able to resolve domain yahoo and rediffmail.. apart of gmail, hotmail, can you send mail to any other provider .. like in.com ?
For yahoo, I know if there is some DNS settings ie. yahoo need to know if you are the authenticated sender otherwise it will mark them as spam.
You should see spam folder in Yahoo.. and for rediff.. wait for a while, sometimes rediff takes ages for a mail to show up.