Single opt-in
The subscriber is added to the list on form submit, without a confirmation-email step.
Single opt-in is a subscription flow where the user submits their email address on a signup form and is immediately added to the marketing list — no confirmation-email step required. The opposite is double (confirmed) opt-in, which requires the user to click a verification link before being added.
Single opt-in maximizes signup-form conversion at the cost of list quality. The list will include typos, form-spam, and people who didn't fully realize they were signing up. List hygiene practices (immediate engagement-based suppression, list validation, double-opt-in for dormant segments) can mitigate these problems but rarely eliminate them.
For most marketing lists, double opt-in is the safer default. For very high-intent flows (paid product trials, gated content downloads where the user explicitly clicked "send me the report"), single opt-in is sometimes defensible because the intent signal is strong enough that the list-quality risk is low. Match the opt-in pattern to the intent strength.