How to warn senders who are about to send emails to a list while adding the list in To field 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 MailTip tab enter a warning or some message.

Step 6: Click on Save.