Email is actually a pretty poor new customer acquisition tool for all the reasons spelled out by previous commenters. The best use of email, in my experience,

Email is actually a pretty poor new customer acquisition tool for all the reasons spelled out by previous commenters. The best use of email, in my experience, is to keep existing users engaged and informed about your company/product. To answer your original question, we send user emails in HTML so that we can include light branding elements (logo, etc), but design them in such a way that they look like text emails. That is, we don't go overboard with images and tables (most webmail clients default to images off anyway). You will also want to create a text alternative for those precipitants that can't receive HTML.