DrByte you were right, the Paypal Fraud Management Filter for unconfirmed address was causing the 11611 error. I deselected this filter and the transaction was processed correctly in ZC.

Eventually I will be adding the Fast & Easy Checkout Mod and was wondering if I need to worry that I am not verifying shipping addresses with the Paypal FMF? If I understand correctly this mod allows customers to process their shopping cart without entering an address. An "account" is created behind the scenes so that order information can be passed back to ZC.

Do I need to worry about the Paypal unconfirmed address FMF?

Thanks again for your help, this is my first ZC site and I am looking forward to my first order!