One thing i'd like to see is links at the top of emails asking the user if they want to change their receive frequency; I know many newsletters already have a "change how often you get these" button, however, needing to go to that page and then enter & submit a form is tiresome and tends to not be worth it when the newsletter isn't that important to you. Indicating that you can change to 'monthly' in a single click/tap is probably much better UX.