You can add previously removed contacts to that same campaign from which they were removed in the first place.

They were not removed completely. In other words, they are part of the statistics. 

They are still in the campaign, but their status has changed. This means that they won't receive any further messages unless you change their status to Active and re-add them to the campaign.

From the Single campaign's page, you will have the recipient's overview with detailed information on each recipient inside your campaign and their progress within the campaign cycle.
From the Recipient list select the desired one/s and in the Set Status dropdown choose Active. 
This action returns the contact back into the sequence, and sending to it should continue per schedule from the last sent phase (PHASE column in the recipient list).
 

Campaigns with the Inbox Rotation function a bit differently. Because of this, some recipient statuses should not be changed. When an email account is removed, disconnected, or has unresolved connection issues for a prolonged period, recipients assigned to that account will get Campaign Stopped status. For such contacts, the sequence will not continue even if the status is switched back to Active. This change may affect campaign status, as it can be stuck in the In Progress, even though all available and scheduled emails are sent out.
Find out more about Inbox Rotation in this article.

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