The postmaster address in Office 365 is used for sending system-generated messages and notifications all to recipients who are outside of your Microsoft Exchange Online organization.

You need to use the Exchange Online Powershell to configure a postmaster address. Here’s how to set a postmaster address in office 365:

Use this syntax in Powershell and mention the email address you want to set as the postmaster:

Set-TransportConfig -ExternalPostmasterAddress <emailaddress></emailaddress>

BY Best Interview Question ON 29 Jun 2020