I've processed a lot of paypal payments on my site. HOwever, I've had 2 now where PayPal gives the "Unconfirmed Address" error.

one customer put through 3 orders in 1 day, the first 2 worked, the 3rd gave the Unconfirmed Address error, and I got another one today.

What can I do to figure out what the problem is?

Thanks,