Lookup for "550 4.5.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (4.5.1)
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.