Lookup for "451 4.6.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (4.6.4)
Conversion with loss performed
This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message.