A delivery failure is a failed attempt to deliver a message to an external site. A large increase in rate of delivery failure can be a sign of a network problem such as a dead DNS server or a remote server timing out on responding to connections.
Delivery failures are recorded in the MTA logs with the logging entry code Q. Look at the record in the file msg-svr-base/data/log/mail.log_current. Example:
mail.log:06-Oct-2003 00:24:03.66 501d.0b.9 ims-ms Q 5 durai.balusamy@Sun.COM rfc822;durai.balusamy@Sun.COM durai@ims-ms-daemon <00ce01c38bda$c7e2b240$6501a8c0@guindy> Mailbox is busy