Lookup for "550 2.5.1"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (2.5.1)
Invalid command
A mail transaction protocol command was issued which was either out of sequence or unsupported. This is useful only as a permanent error.