What is single opt-in (SOI)?

Single opt-in (SOI) is a subscription method where a user is immediately added to your email list after submitting their email address through a signup form-no additional verification step required. The subscriber completes one action (filling out the form and clicking submit), and they're instantly part of your audience. This approach prioritizes speed and simplicity, removing any friction between the moment of interest and list inclusion. Many marketers favor SOI because it maximizes signup completion rates and captures subscribers at their peak moment of engagement.

The SOI process typically works as follows: a visitor enters their email address in a signup form, clicks submit, and receives a welcome email confirming their subscription. Unlike double opt-in, there's no intermediate step asking the subscriber to click a confirmation link. The welcome email may be triggered immediately or after a short delay, but the subscriber is considered active on your list from the moment they submit the form, regardless of whether they open or engage with subsequent messages.

While SOI offers simplicity and higher initial signup numbers, it comes with trade-offs. Without email verification, your list may include typos, fake addresses, spam trap hits, or malicious signups from bots or competitors. These issues can harm deliverability over time and inflate your subscriber count with non-engaged or unreachable addresses. SOI gets people on your list faster, but whether they're the right people, and whether they actually wanted to join-remains unverified until they engage or bounce.