Double opt-in is a subscription method in which, after filling in the form, an email is sent to the address given, containing a link with which the person confirms the subscription. Without confirmation, they are not added to the database. It brings three advantages. It filters out typos and non-existent addresses, which protects deliverability. It prevents someone from subscribing someone else's address, which tends to be a source of complaints. And it provides a record of when and from where the consent arose, which is the strongest evidence in a data-protection check. The cost is a lower number of subscriptions, since some people do not complete the confirmation. This drop-off is, however, only apparent – unconfirmed addresses would not have opened emails anyway. Send the confirmation email immediately, therefore, with a clear subject line and a single button.
See also: Contact list hygiene, Legal basis for email marketing, Email Deliverability.
