Single opt-in (SOI) is a subscription process which does not require any confirmation saying that the owner of the email address willingly opted in. In this approach, the email address of the owner is added to email list right after he fills out the subscription form.
Double opt-in (DOI), also known as confirmed opt-in (COI), is a subscription process which requires the email owner to click a confirmation link in the email sent after susbcription activity.
+ Build a large list quickly + No contact leakage
- Potential for low quality, unengaged subscribers - More time spent on list maintenance - Higher risk of spam complaints and lower email deliverability
+ Cleaner list + More active, engaged database
- Slower list growth - Potential for contact leakage
A study conducted by Litmus: email marketers from all around the world have answered the question of "Which permission method do you think is better?: Single Opt-in or Double Opt-in". The result was like that; 51% of respondents said Single Opt-in is better and 49% of respondents said Double Opt-in was better.
According to Popupsmart email marketing specialists, double opt-in formed email list building technique is better because having high-quality subscribers who willingly opt-in to your email list is more beneficial than having lots of subscribers who may mark you as spam.