Double Opt-In/Confirmed Opt-In

Asking for permission is key. A double opt-in, or so-called Confirmed Opt-In is the recommended practice for subscribing recipients to your potential email list or newsletter. 
Let's say someone wants to subscribe to your newsletter and they request an opt-in. 
A double opt-in implies that the next step is sending a confirmation email message to that specific contact. 
This way, you can automatically ask for confirmation and your visitor can actually verify that they have in fact decided to subscribe to your future newsletters.