Please check the Junk or SPAM folder to see if it has landed there?
If it is not found, then the problem has to be addressed to the IT team of the domain to which you are mailing.
They will have to check for calls coming from sending IP of your sandbox. It is possible that there is internal filtering of email on your server. At times, this also happens because of proxy servers on the target domain side.
If the delivery log is showing Sent, it means that target domain server has acknowledged our email and there is nothing much that can be done from campaign side.