If the broadlog shows as email sent but it is still not received by the end user then deeper investigation needs to be carried out. For example reviewing the MTA child logs to see if mta processed the email or not and next course of troubleshooting would depend on it.
As you have mentioned its a organization sandbox so either campaign support team or spp team would be able to assist you in this case. I would suggest to raise a ticket with support team providing details like instance url , delivery name so the team can start investigating the mtachild logs which are not visible via the console itself.
Can you also check the sending logs to verify if that shows as sent ? At the same time if the email was sent within your organization , Can you ask the recipients to check if the email has not landed in their spam/junk folder ?