Zervant sends the invoices through an email account email@example.com. If your customer claims that they have not received your invoice, please ask them to check their spam folder and to add firstname.lastname@example.org as a trusted sender. There is a chance that the receiver has high-security settings in their email account that blocks noreply@-email addresses.
You can also follow up on the current status of your invoice under the tab >Invoices by selecting the invoice and by clicking the tab >History on the bottom right. If an error occurs in the email delivery, you will also see a short explanation of why it failed.