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Soft bounces are managed automatically and unsubscribed after N bounces. If you want more details on how this happens, please see this article from the MailUp Knowledge Base: http://support.mailup.com/KB/a263/how-managing-bounces-affects-reputation-and-deliverability.aspx?KBSearchID=8729online guide.
If you wish to, you can unsubscribe soft bounces manually, faster than what MailUp does automatically (this is typically not required). To do so, under Recipients > Channels > Email > Bounces, use the Recipient management feature (at the bottom of the page - view screenshot) and move them to Unsubscribed.
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