The external postmaster address is used as the sender for system generated messages that are sent to users outside the organization. All domains which are not in the accepted domain’s list are treated as external domains. By default, the External Postmaster address is set to $null. Though it is set to $null, the behaviour is different. It doesn’t mean that there is no email address at all. The transport servers use Postmaster@defaultaccepteddomain as the external postmaster address if it is not explicitly set by the administrator.
This will work fine for organizations that have their default domain as their external domain name (externaldomain.com rather the AD domain.local). But, if you have your AD domain as your default accepted domain, you need to change the postmaster address to an external routable address. As per RFC standards, all domains should have a postmaster address configured and should be able to receive emails that come to that address. Hence you either need a mailbox for the postmaster or add the email address to an existing account.
Running Get-TransportServer gives you the current status of external postmaster address.
Set-TransportServer cmdlet have to be used to set the external postmaster address. Run
Set-TransportServer –identity “transportservername” –ExternalPostmasterAddress “emailaddress” This command will only set the postmaster address on the transport server specified.
If you want to set the address on all hub transport servers, run
Get-TransportServer | Set-TransportServer –ExternalPostmasterAddress “emailaddress”
If you have edge transport servers in your environment, the postmaster address has to be set on a per server basis using the Set-TransportServer cmdlet (you can’t pipe).