Email newsletter is the most popular way to inform users about new daily-deals. But with anti-spam filter rules getting more and more strict, it becomes a challenge to prove that your newsletter is legitimate, and each recipient was actually subscribed to it.
For Getsocio daily deal newsletter we have added an optional "double opt-in" mode. It can be enabled via "Confirm subscription" toggle in "Email" block on the "Settings" page. When enabled, each new subscriber will have to click a link in the email to confirm they actually want to receive the newsletter. And if they do not click, they will not be bothered by email again.
While this option is off by default, it may be reasonable to enable it on your site to be sure that your subscribers are indeed happy to receive your newsletter.