6Jun/080
Why is IPN Responding with ‘INVALID’?
If you're doing an PayPal IPN implementation, you should be careful to set the right encoding for IPN (You probably want UTF-8) - otherwise international characters will be read in one form from the "subscr_signup", "subscr_payment" or "web_accept" form data and written in another form in the "_notify-validate" response - which will make the validation fail.