One of the functionalities in Microsoft Exchange for Distribution Groups (or mail-enabled groups for that matter) is ability to setup approval workflow. This means you can require any message to be manually approved before it's delivered to user mailboxes. It's a standard functionality for Microsoft Exchange and generally works out of the box. Until it doesn't… Of course it doesn't stop by itself. It's strictly related to Exchange On-Premise in a hybrid scenario with Exchange Online and it manifested itself when some people were moved to Exchange Online, while another group stayed on-premise. There were simply no Approve / Deny buttons in the message that was sent to Approvers.