If a contact opts out from emails, are they deleted from my contacts/contacts lists? Or are they added to a opt’d out list? if in the first case I re-add the contact, does the contact still receive the email?
Contacts that opt out of email are not removed from the portal, they just get marked as ineligible for email sends. You can export a list of contacts that have opted out of email using the “Export unsubscribes and bounces” link in the lower left of the Content > Email dashboard.
The opt out status is stored at the email level, so if you delete the contact record and create a new record with the same email, that new record will be marked as ineligible.