You can customize the email templates used here, just like all other email templates, under Settings → E-Mail-Management → E-Mail-Vorlagen (Email Templates). You'll find them under Benutzer-E-Mails (User Emails). You can find a general guide from Shopware here.
Note: When customizing email templates, the sender address that is used by default is the one stored in the translation of the template for the respective subshop.
In the Shopware default setup, the variable {config name=mail} is stored here, which refers to the email address stored for the respective subshop under Settings → Grundeinstellungen (Basic Settings) → Shopeinstellungen (Shop Settings) → Stammdaten (Master Data) in the Shopbetreiber E-Mail (Shop Operator Email) field. So if you manually enter an email address in the Absender (Sender) field in the window shown below, this will not appear as the sender of the email. To make the desired email address appear, click on the globe icon to go to the Übersetzungen (Translations) window.
There, you can select the desired subshop from the available languages on the left and enter the desired email address in the Absender (Sender) field.
Small text snippets can be edited under Settings → Textbausteine (Snippets) in the Textbausteinverwaltung (Snippet Management). To do this, enter the term viison_dunning_document in the search bar. You can find out more about snippet management in this article.