Configure the external postmaster address in Exchange Online
The external postmaster address is used as the sender for system-generated messages and notifications sent to message senders that exist outside your Microsoft Exchange Online organization. An external sender is any sender that has an email address in a domain that isn't configured as an accepted domain in your organization.
By default, the value of the external postmaster address setting is blank. This default value sets the external postmaster address to the value postmaster@<Default accepted domain> for your organization.
There's no mailbox associated with the postmaster@<Default accepted domain> email address.
What do you need to know before you begin?
Estimated time to complete: 15 minutes
You need to be assigned permissions before you can perform this procedure or procedures. To see what permissions you need, see the "Mail flow" entry in the Feature permissions in Exchange Online topic.
You can only use Exchange Online PowerShell to perform this procedure. To learn how to connect to Exchange Online PowerShell, see Connect to Exchange Online PowerShell.
Use Exchange Online PowerShell to configure the external postmaster address
To configure the external postmaster address, use the following syntax:
Set-TransportConfig -ExternalPostmasterAddress <EmailAddress>
This example sets the external postmaster address to the value
Set-TransportConfig -ExternalPostmasterAddress email@example.com
This example returns the external postmaster address to the default value.
Set-TransportConfig -ExternalPostmasterAddress $null
How do you know this worked?
To verify that you have successfully configured the external postmaster address, do the following:
Run the following command to verify the property value:
Get-TransportConfig | Format-List ExternalPostmasterAddress
A blank value indicates the default value postmaster@<Default accepted domain>.
From an external email account, send a message to your Exchange organization that will generate a non-delivery report (also known as an NDR or bounce message). For example, you can configure a mail flow rule (also known as a transport rule) to send an NDR for a message from that sender that contains specific keywords. Verify that the sender's email address in the DSN matches the external postmaster address you specified.