Lookup for "251 4.1.8"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (4.1.8)
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.