Lookup for "551 2.1.8"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (2.1.8)
Bad sender's system address
The sender's system specified in the address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the "@" is invalid for mail.