If you are looking to send out Commercial Electronic Messages, then you need to get consent, either implied or express. Double Opt-in is a great method.
I always recommend aiming for Express Consent, and using the double Opt-in method.
Double Opt-in is a two-step process where people interested in your communications fill out a sign up form, and then receive an email with a link to confirm the subscription. I’m sure you have had this happen to you, you visit your favorite website you sign up for the newsletter, and after it says check your email to confirm. That is the double opt-in method as you chose to sign up and then you confirmed it via email.
The great thing about this, is that you create a far more valuable list as your members were willing to invest a bit of their time to be able to get your communications. These types of lists typically have a much higher engagement, which means more opens and clicks, and fewer bounces and unsubscribes.
This also provides protection for you from scammers, bots or people who sign up other people’s email addresses, as they won’t start getting messages form you until they confirm the opt-in request. It’s unfortunate, but the last thing that any of us would want is to be sending our newsletters to a list that was created by spam bots, and not reaching people who actually wanted to read our message.