If one of your subscribers opts out of your email marketing and later wants to resubscribe, they can do so by updating their email subscription preferences.
Please note: HubSpot is unable to resubscribe contacts on your behalf. The recipient must take action themselves in order to receive emails again.
The easiest way for a contact to opt back into email marketing is to locate an email they've received from you in the past and update their email subscription preferences:
Another option is resubscription emails. With resubscription emails enabled, contacts can also resubscribe to your emails by returning to your website and filling out a form or lead flow. This feature notifies unsubscribed contacts of their opt-out status when they enter their email address on a form or lead flow and allows them to resubscribe. To learn more about this feature, check out this article.