Emails classified as "Dead Letter" are unable to be delivered because the email generated an error in delivery. This means that something in the email (attachments, or the coding of the email) was unable to be scanned by the mail system for viruses etc. This could also be if the email was corrupted in some way during delivery and is unable to be opened.
Sometimes adjusting an attachment or having it saved in a different format can fix any issues in delivery.
If you would like further assistance, please forward the "Message Digest" email, which includes the details of the "Dead Letter" email in question, to email@example.com