An email which they sent to someone at the domain listed in the message has been delayed for one reason or another.
It could be that the mail server which is to receive the message is down, the message could be stuck in a queue waiting to be processed etc.
There isn’t anything you can to do stop them. Once the message has been delivered or times out the messages will stop.
The MX record for the domain could be pointing to an incorrect address. The host continues to attempt delivery to this bad address. Be sure that your DNS is configured properly and that the destination address is correct.