Double opt-in means a signup isn’t a subscription until the person clicks a confirmation link you emailed them. Single opt-in trusts the form; double opt-in verifies the inbox.
What it buys you
- Proof of consent. The confirmation click is timestamped evidence that the actual owner of the address said yes—the strongest position you can be in under GDPR-style consent rules and Canada’s CASL, and the cleanest answer when a complaint dispute arises.
- A cleaner list at the door. Typos never confirm. Bots rarely confirm (pair it with a CAPTCHA and they mostly stop arriving at all). The address that confirms is valid, reachable, and belongs to someone interested enough to click—which is exactly the engagement baseline inbox providers reward.
- Fewer landmines. The person who didn’t sign up—mistyped address, prank entry, scraped form—never enters your list, never gets your campaign, never files the spam complaint.
What it costs
Friction. Some genuinely interested people won’t complete the confirmation, and your list grows slower than your signup form suggests it should. That’s the whole trade: a smaller list of people who demonstrably want your mail, versus a bigger list carrying unknown risk.
For deliverability, the smaller list wins. Complaint and bounce rates—the numbers that decide your inbox placement—punish quantity without consent.
Where it tips
- Marketing lists and newsletters: default to yes. This is where complaints breed and where consent proof matters.
- Account-based products: creating an account with email verification already is a double opt-in for transactional mail. You don’t need a second ceremony—but marketing consent should still be its own unchecked, specific checkbox, never bundled into the terms of service.
- Markets where it’s expected: in much of Europe (Germany especially), confirmed opt-in is customary bordering on assumed. Follow the custom.
If your complaint rate is comfortable and your acquisition is clean, single opt-in can work. But if you’re asking, the safe default is the one with the receipt.