Assign a moderator to approve all the emails sent to a distribution list in Office 365

Step 1: Sign into Office 365 admin portal via https://portal.office.com

Step 2: Click on Admin from the left pane and navigate to Groups > Active groups.

Step 3: Click on the list you want to assign a moderator.

Step 4: Switch to the Settings tab and click Manage more settings in the Exchange admin center.

Step 5: Switch to the message approval tab and click Messages sent to this group have to be approved by a moderator.

Step 6: Under Group moderators click on add icon and select a moderator.

Step 7: Click on Save.