Lookup for "450 4.6.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
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.