While testing mail flow I noticed that when an NDR was sent out by my Exchange 2013 the default postmaster account was used, with the default domain, i.e. postmaster@contoso.local (I use a .local TLD on my Active Directory domain) instead of a regular and routable SMTP domain.

To change this you can open the Exchange Admin Center, navigate to Mail Flow | Receive Connectors and click the more options (the three dots) and select Organization Transport Settings as shown in the following figure.

Select the Delivery option and enter the External Postmaster Address in the appropriate box:

It is also possible to do this in the Exchange Management Shell using the following command:

Set-TransportConfig –ExternalPostmasterAddress postmaster@contoso.com

Next time an NDR message will be sent using the postmaster@contoso.com address.