3491923 - Why is there an undeliverable email notification under the Messages section of my Guided Sourcing Event?

The message is failing to deliver emails to customer and we can see an undeliverable email in the Messages section of my Guided Sourcing Event;

When opening the message it will have a message similar to the below:

"Event Title : <Event Name>
Original Recipient: <Full name of the supplier/buyer user that email had fail to be sent>
Original Subject : <Subject of the email that failed to be sent>

Bounced Message
--------------------------------------
This is the mail system at host <Ariba email server name>

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

: Host or domain name not found. Name service error
for name=<domain from email failed> type=A: Host not found"

(In red is the information of the error).

KBA , BNS-ARI-GS-EVT-NOT , Guided Sourcing Event Notifications , Problem