There are several types of people you cannot add back to your list.
People who have bounced off: If someone was previously on your list, and we received enough bounce messages that they were removed from your list, they cannot be re-added using that address.
People who have unsubscribed themselves: If someone clicked the "Unsubscribe Now" link, or unsubscribed themselves from the member page, or sent an email to the unsubscribe address, you cannot re-add them.
People who have reported your list messages as spam: You cannot resubscribe someone who has reported your list messages as spam.
Resubscribing these people is a violation of our deliverability agreements with major ISPs. Additionally, sending mail to bouncing addresses, or having people report your list messages as spam drastically increases the likelihood that your list messages will be discarded as Spam or Junk Mail for all your subscribers, instead of being delivered to your legitimate subscribers' Inboxes. So, keeping off bouncing addresses and people who do not wish to be subscribed to your list should result in a higher open rate from the subscribers who do want to receive your list mail.
If one of your subscribers says that they accidentally unsubscribed themselves or reported a list message as spam, they will have to resubscribe themselves. To do so, they should go to the member page at http://www.listbox.com/member.
Listbox © 1996—2017. The Listbox Name and Logo are registered trademarks of FastMail Pty Ltd