The undeliverable email detection feature is extremely useful for proof owners so they can immediately know if the reviewers they have invited to a proof did not receive the invitation email. Most often, the cause of this notification is a mis-typed email address, or their email inbox is full.
If you have received an undelivered email notification:
Open the proof’s info pane and double-check the spelling of the email address of the reviewer.
If the email address is incorrect, in the workflow step add the correct email address of the reviewer in the anyone else? field. Once you have added the reviewer, click the 3 dots icon next to the incorrect email address and remove the reviewer.
This article on adding and removing reviewers will be handy too.