When you have a robust list of free registered users, you should routinely try to upsell them into a premium membership. Marketing Profs does this nicely with this email. The image gives a witty take on the upsell, making it more engaging than a boring offer. The use of a code encourages word-of-mouth marketing (as opposed to offers made strictly to registered users). Also, the email contains a link for a free trial, but it's subtle ("Sample some PRO content") thus making sure that the emphasis ...