When an email is primarily accepted by the MX server for mailbox delivery and if it is bounced later then it is treated as “Soft bounce”. Many times, the bounce copy of such emails is not shared instantly in the correct format by the MX server of the recipient domain due to which the exact reason is not showcased in the panel. 

At times, the emails are accepted by the recipient MX servers initially however later they are bounced due to specific issues with email delivery for the same email id. In that case, the bounce copies shared the recipient MX server are not in human-readable format due to which these emails are shown as bounced status on the panel with remarks as "unknown reasons"

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