If I want to change font in the emails, is it related to discourse or the mail service?

[Falco]

This should work:

Can I customize the email 'container' html too?

There’s a string with id user_notifications.header_instructions that can be used to add some html to the top of all HTML emails. It can be customized from Admin > Customize > Text Content. Here’s a simple example that would display a centered image (like a logo) with a red line above the email’s content: <p style="text-align: center; border-bottom: 2px solid #B90B26; padding-bottom: 15px; text-align:center;"> <img src="...absolute-url-to-an-image..." width="90" height="66"> </p>