The limitation on what you can do within your HTML in your is mainly based on what your and your receipents mail client understands. That is why using custom fonts and CSS stylesheets will most likely not work across our ecosystem of people you send email messages. Even if your mail client would support custom fonts or CSS stylesheets the likeliness that this will not be the case for at least some of your recipients is great.All our customers are free to try out what works and what does not. From a product standpoint, we do not limit any HTML options. Whatever you define in your email template we will hand it over to Outlook to insert. Outlook then decides how to handle your HTML.