If you check "Send as HTML", you can use these HTML tags in your message:
<h1>, <h2>, <h3> - Headings<p> - Paragraphs<b> or <strong> - Bold text<i> or <em> - Italic text<u> - Underlined text<a href="https://example.com">Link text</a> - Links<ul><li>Item</li></ul> - Bullet lists<ol><li>Item</li></ol> - Numbered lists<br> - Line break<hr> - Horizontal line