As Admin you can delete specific wrongly send emails from your users mailbox :
Open Exchange Powershell , To delete from user@example.com email with the subject “Mail delivery failed: returning message to sender” type :
Search-Mailbox -Identity "user@example.com" -SearchQuery 'Subject:"Mail delivery failed: returning message to sender"' -DeleteContent
To delete email from specific user “admin@inyourderam.com”:
Search-Mailbox -Identity "user@example.com" -SearchQuery 'From:admin@inyourdream.com' -DeleteContent