The most common method of sending an invoice to your customer is sending an invoice via email. Send Invoice feature helps you save time during create emails by open your default email client, attach the PDF file, and provide a default subject and message. Email field in your customer details must be filled with correct email address. Feature works great with Microsoft Outlook and also with each eamil client which recognize .eml files.
Unfortunatelly, Windows 10 mail client doesn't work with this feature properly because does not recognize email as outgoing message. You can only see message preview without possibility to send it. Maybe Microsoft change it in the future.