Hi @Markus_Reinecke, thanks for getting back to me. Yes, you can point the form to your mail script url by putting the url to the mailscript file in the Action field. Set the form Method to POST.
Take peek at this article on working with exported forms: http://help.webflow.com/elements/forms#exported-forms
After you export the form, when the form is submitted, the url in the Action field will be called and the form fields posted to the mail script.
Webflow does not support external mail scripts, but there are examples all over the web, plus it is possible for you to share your code here to get help 