Cannot email invoices

[Francis Hunt]

When I first installed Express Invoice I set up 3 customers to receive invoices by email and these still work.

However I cannot get Express invoice to send email invoices to any additional customers that I add. The send bar delivers an error message half way through sending. If I substitute the email address with one of the original 3 addresses (after advising the customer first) Then the invoice goes, but of course to the wrong customer. Any Ideas. I have tried a variety of email addresses including my own