Lookup for "504 2.6.4"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (504)
Command parameter not implemented
Enhanced Status Code Details (2.6.4)
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.