To send an invoice to a client or a contact, open the Invoice you want to send out and click Email invoice.
This will open up a window where you can choose from your Invoice email templates, recipients and change the email title. You can edit the content of the email from this box as well.
Once you've emailed your invoice, you will see the status change from Draft to Awaiting Payment.
If you're using the Stripe integration, ensure your Invoice email template in Settings has an invoice link, so your clients can pay their invoices online.
The status of your email will be shown as:
sent
delivered
viewed
failed
You can filter for invoices that may have failed to email in the invoices tab.
If an invoice has failed to send, check that the email that you are sending to is correct and then resend.