What is single opt-in (SOI)?

You set up a signup form. Someone types their email and clicks submit. Thirty seconds later, they're on your list and your welcome sequence fires. That's single opt-in: one step, no confirmation required, and they're in. It's the fastest way to grow a list, and it comes with tradeoffs worth understanding before you decide it's the right approach for you.

The main appeal of single opt-in is conversion rate. You don't lose subscribers at a confirmation step, which can account for 20 to 30 percent of people who'd otherwise complete the process. If you're running a time-sensitive lead magnet or a high-traffic landing page, that friction reduction matters. Your list grows faster, and for some businesses, speed genuinely is the priority.

The tradeoff is list quality. With no confirmation step, you'll collect typos, fake addresses, and sometimes addresses entered by someone other than the owner. Those bad addresses cause hard bounces, which hurt your sender reputation with inbox providers. You're also more exposed to spam trap addresses, which can damage deliverability if enough accumulate. This is why single opt-in lists typically need more active list hygiene over time than their double opt-in counterparts.

From a compliance standpoint, single opt-in is acceptable under CAN-SPAM and is common practice in the US. Under GDPR and CASL, you'll need to be more careful. The consent you collect needs to be clear and documented, even if a confirmation email isn't required. Many senders add an explicit checkbox to their SOI forms specifically to create an auditable consent record for subscribers in regulated markets.

If you're unsure which approach to use, think about your acquisition context. Single opt-in works well when you have strong brand recognition, a clear signup situation (like a checkout form), and a list hygiene routine you're actually running. If you're acquiring emails from cold traffic or paid lead gen, double opt-in will save you deliverability headaches later.