Lookup for "501 2.5.4"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (2.5.4)
Invalid command arguments
A valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. This is useful only as a permanent error.