Single opt-in and Dual opt-in, what is the key difference?

[william_l]

In simple language

Single Opt In = a new email subscriber signs up on your website and provides their email address and then immediately begins receiving your email marketing messages

Double Opt In = a new email subscriber signs up on your site and provides their email address, but must FIRST go to their email account and click a "confirmation link" before receiving any of your email marketing messages