Simple Invoices - Forum

I have been using Simple Invoices for a while and all was fine. But today, I tried to email an invoice to a customer and received this error: "Zend Mail Protocol Exception: 5.7.1 : Recipient address rejected: Access denied " I checked that the email username and password are correct in the config file.

version.name = 2010.2 update 1 - stable

Any ideas?