I’m having the same problem with “mc_gross”. Sometimes it’s sending the wrong value to PayPal and then Magento classifies the order as “Suspected Fraud"… Anybody found the cause and solution?
I was having this error of “Suspected Fraud” when testing with the Paypal sandbox. I noticed that the field I was sending “mc_gross” did not match my order total in Magento. I changed this to match and resent the IPN and eveything worked fine. the order was moved to processing.
Have a look at the file app/code/core/Mage/Paypal/Model/Standard.php to see which IPN fields are important to Magento. The function ipnPostSubmit() cares about mc_gross and payment_status=="Completed".
You could temporarily modify this file to get more debug. Make sure debug is turned on for Paypal. Goto System>Configuration>Paypal and in the tab Website Payments Standards Setting make sure Debug Mode is “Yes”
The order comment history also gives information. Go to admin>sales>orders and pick an order and look at the history.
I was seeing comments like
The successful comment looks like this
1 Apr 2011 06:51:15| Suspected Fraud |Customer Notified
IPN "Completed". Order is suspended as its capture amount €0.00 is suspected to be fraudulent. Transaction ID: "541545".
1 Apr 2011 07:16:37|Processing
Customer Notification Not Applicable
IPN "Completed". Registered notification about captured amount of €0.66. Transaction ID: "541545".