WordPress: Contact Form 7

Your Contact Form 7 forms keep working as they are; people who tick a newsletter box on them are also added to 404 Emails. Only their email address and name are sent - never the message.

1. In Contact → Contact Forms, open your form and add a newsletter box, for example: [acceptance newsletter optional] Yes, send me the newsletter [/acceptance]. Save.
2. Install the 404 Emails plugin: download it under Settings → Integrations → WordPress in 404 Emails, then in WordPress go to Plugins → Add New Plugin → Upload Plugin, choose the file and click Activate.
3. Open 404 Emails in the WordPress menu, click Connect to 404 Emails, sign in, choose your workspace and click Allow.
4. Go to 404 Emails → Other forms. Next to your form, choose the 404 Emails form in Add sign-ups to, and in Who is added choose Only people who tick "Yes, send me the newsletter". Click Save.
5. Send the form once with the box ticked and check that the person appears in Contacts.

Only people who tick the box are subscribed. The wording they ticked is saved in 404 Emails as their consent.

A form without an email field can't add anyone. A privacy-policy box isn't permission for newsletters - add a separate one.