If you have your own triggered campaign set up under 'Double opt-in settings' in Contacts > Advanced features > Subscription settings, then this can be used on contacts already in your address book(s) who don't have an opt-in type of 'Double verified'.
Using double opt-in in this scenario will help you be sure that a contact wants to receive your email, and will provide you with proof that they do. It's also best subscription practice in email marketing for these very reasons, guarding against possible subscription abuse.
Editing your triggered double opt-in confirmation campaign
You may wish to firstly edit the wording of the triggered double opt-in confirmation campaign you're sending to contacts if they didn't subscribe to your address book via a signup form on your website. For an example, see below.
Please note: External dynamic content and advanced personalisation can't be used in triggered double opt-in campaigns.
Steps to apply double opt-in against a contact in an address book
1. Select Contacts from the navigation bar and then select the address book you require to bring up the contacts list.
2. Click on the Edit pencil icon alongside the appropriate contact to access the 'Manage contact' screen.
3. Mouse over the 'Opt-in type' tile and click on the link to open the 'Edit' side panel.
4. Under 'Opt-in type', click on the link to open the 'Opt-in type' panel.
5. Select 'Verified double' and click Apply.
6. A confirmation message will inform you that the contact will be sent a verification email, that they'll be temporarily removed from your address books and that any non-transactional email you send to them will also be temporarily stopped. Click Save and send email.
What happens next
The triggered double opt-in confirmation campaign automatically gets sent to the contact. As explained above, they'll be temporarily removed from the address book until they've clicked on the double opt-in verification link.
Once the verification link has been clicked, the contact will be added back to the address book and their opt-in type will be updated to 'Verified double', as shown below.