Hi @Jayden Tan Pek Tung ,
An undeliverable mail or NDR (Non-Delivery Report) is a message that is generated and sent by the mail server to inform the sender that the email delivery was not successful. So normally it's not recommended to block undeliverable mails.
Anyways, if you do want to stop seeing these particular mails with the title "Undeliverable: Thank you for your submission!", you can create an inbox rule similar like below:
In this way, incoming mails with this title will be automatically deleted.
You can also click "Run Rules Now" to clean up all existing mails in your Inbox with this title:
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