How to allow external users to send emails to a 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 check the Allow external senders to email this group.

Step 5: Click on Save.