Hello @Leah007 ,
Our system keeps track of what contacts unsubscribe from you, and prevents them from being sent emails, even if you try to re-add them to a list or re-upload them. The only way they can be re-added is if they explicitly sign back up through one of the resubscribe options:
the resubscribe email that you can send
them using one of your signup forms - e.g. a landing page form
you getting an email with explicit permission from them to resub, then you working with our Review team for a manual resub
If you're asking why we strongly recommend retaining those records for your own database outside of us, that's because an unsubscribe is considered legally-binding. This is regardless of what email service provider you use at the time. So if you move to a different service, or choose to send a marketing email directly from your address, we want you to have access to the unsubscribe info so you're being compliant with the contacts' permissions.
If you have additional questions on how unsubscribing works, make sure to check out our main article on the topic.
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