Settings → Email → Do-not-email list: the app never emails these addresses: people who unsubscribed, addresses that bounced, people who marked an email as spam, and anyone you add. Automations always respect it.
What each one stops
- Unsubscribed: stops marketing emails only. Replies to them and confirmations they asked for (transactional and one-to-one emails) still go. Their email permission shows as unsubscribed.
- Bounced (the address doesn't exist) and marked as spam: stop every email.
- Added by you: stops every email.
You can take someone off the list with Remove ("Let the app email this address again?"), except people who marked an email as spam: Can't be removed: they marked an email as spam. That protects the business's sending for everyone.
The unsubscribe link
Every marketing email has an unsubscribe link at the bottom, next to the business's postal address, and the header inbox apps use for their own Unsubscribe button (one click). Either one unsubscribes the person straight away, and the page lets them undo it if they clicked by mistake. The unsubscribe goes on the contact's Activity history and can start an automation (Unsubscribed).
Do not contact (on the contact's page) is separate: blocking Email there also stops every email to that contact.