What is the optimal way to accomplish this task?

It would be easy for people to place their email addresses to sign up for newsletters but the thing is, not everyone is actually willing to receive newsletters. They may input a fake email address to supposedly get a coupon that they will just use once.

A double opt-in will require people to validate their email addresses. This will also mean that they are determined to receive emails from the company. They can also input a code that will make the company know that their email address is valid. This way, companies will not make any effort to send emails to fake and inactive email addresses.