System not recognising customer email addresses?

When I select invoices from the "Sales/Invoices" screen, and click "Email", I get the little yellow exclamation mark icon and an error message stating that "One or more selected invoices don't have a customer email" however all of these customers DO in fact have an email saved in the customer card. If I drop into an individual invoice and select "email invoice" the system immediately recognises there is indeed an email address and opens the email pop up window, all tickety boo. Does the bulk email function not work? Is there a setting somewhere I need to change?

Thanks