Generally, the company email address is used for sending invoices. You can however also set a specific email address only for sending invoices.
Here is how to set or change it:
Navigate to Settings and click on Company information
Click on your company entity to expand it and choose Edit
In 'Company details', fill out the field 'Invoicing email'
Click on Save and close
All invoices sent from your company will be sent from this email address.