You may see this error message under “detailed fax status field” on sent fax record and fax status is “not delivered.”

Reason for this error:
A retrain negative (RTN) message is a result of discarded data during the fax transmission due to interpage timing protocol. If a retrain message is received during the fax transmission eFax Developer will propose a new bit rate. It is up to the receiving modem to accept the newly proposed bit rate or ask for another retrain sequence. If both parties can not negotiate a bit rate the result will be “One or more pages was confirmed as RTN” failure message.

To resolve: This should be investigated by the other fax carrier that is returning this error message.