When you use your own iOS mail app and it isn’t configured correctly then the send button is grey or you receive an error message "Your device is not configured for sending of emails."
How to solve it
Navigate to ⚙️Settings - Client communication - Email settings
There are two options:
Leave the switch enabled to you use your own mail app.
Disable the switch and you will use Billdu to send your emails. Learn more about this option.
Another option - sharing, messaging
Share your invoices as a message, with Whatsapp or any other app as a link.
Take a look here to learn more about sharing an invoice.