Answer: The Exchange Postmaster address is used to notify about any email sent to an email address that does not exists in the organization.
To change or configure the Postmaster address we can either use the GUI or the shell interface.
To configure The Postmaster Email Address Follow the Steps Below:
Open EMC -> Organization Configuration -> Hub Transport -> Send Connector Tab
Double click on the Transport settings
In the general transport settings tab type the postmaster address under External Postmaster Address.
You can use the Exchange shell by using the following cmdlet:
Set-TransportConfig -ExternalPostmasterAddress firstname.lastname@example.org