Single opt-in (SOI) is a subscription process which does not require any confirmation from the owner. In this approach, the email address of the owner is added to an email list right after they fill 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 susbcribing.
+ 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 answered the question "Which permission method do you think is better?: Single Opt-in or Double Opt-in". The result was: 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 lists' building techniques are 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.