Single Opt–In Campaign

A single opt-in campaign means that as soon as a user completes an enrollment form for a campaign, his/her subscription to the campaign is confirmed. Unlike a double opt-in campaign which confirms the user's e-mail address and improves reader responsiveness, a single opt-in campaign in only recommended when you are certain your users wish to receive the content of the campaign you are sending.

Read more about double opt-in campaigns for a more thorough comparison.