Lookup for "214 5.5.1"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (214)
Help message [Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user]
Enhanced Status Code Details (5.5.1)
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.