Chances are, the receiving email server is accepting the campaign but preventing it from being delivered to the inbox.
As long as the recipient is at a private/business domain, they can contact their IT team to white list our sending domain or IP range. White listing either of these on the email server will then allow campaigns to be delivered.
IP range: 18.104.22.168/23 (141.193.208.[0-255] AND 141.193.209.[0-255])