Change e-mail font type

Where can we see the website in question?

It concerns the form @ Aanmelden
@ https://huisvoorlevenskracht.nl/training-cursus/kom-koken-in-de-polder/

See Setting up mail

What email settings do you have in the Mail tab panel?

To protect personal data, hiding email addresses in the settings or replacing them with dummy addresses is strongly advised. You can also include screenshots.

Just the [field] stuff.
For what I read, there should be a default .css in which I can change the font type of the e-mail to be sent. Can someone tell me which one that is?