If you send an email marketing campaign with HTML formatting, you have the opportunity to grab your reader's attention by applying formatting to your copy. To emphasize important points you should stick to bold face.
Italics, the way to emphasize in printed text doesn't work as well on a computer screen, and underlined text looks bad and is hard to read, on-line or off. Additionally, clickable links are traditionally underlined, and non-clickable underlined text may confuse some readers. That's why you should use bold to emphasize.