You'll probably hear the words "double-opt-in" bandied about a load over the next few weeks. It's because of GDPR.
Basically it means that it's not enough for someone to sign up for your mailing list. That's a single opt-in. They then need to confirm that they really want to be on that mailing list. That's the second, double, opt-in.
The reason is simple - anyone can go to a sign-up form and put firstname.lastname@example.org in there, instead of their real address. Snappy's might think your newsletter is great, but they haven't asked for it. Which effectively means you're spamming them.
A double opt-in prevents this, as Snappy's have to confirm a second time that they really want your stuff.
Keeping them, and you, safe.
Take action: Are you using double opt-ins on all your email sign-up forms?