IPN Currency Issue

Double check if the account (including sandbox account) accept the currency or not. If your account didn't accept the currency, IPN will receive "multi_currency" error. Here is the sandbox account console:

https://paypalmanager.sandbox.paypal.com/login.do

Detailed solutions:

http://www.webhostingtalk.com/showthread.php?t=729276

Official explanation:

https://www.paypalobjects.com/en_US/ebook/PP_OrderManagement_IntegrationGuide/ipn.html#1070711

Supported Currency:

https://cms.paypal.com/mx/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_formbasics