What can I brand?
In Settings → Brand & documents: "Logo, colours, document language and invoice layout." These appear on your invoice and quote PDFs and on the customer page.
Why is the line in the email?
It tells your customer who sent the email and through which service. Your business name always comes first.
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.