Lookup for "251 5.1.1"
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 (251)
User not local; will forward to
Enhanced Status Code Details (5.1.1)
Bad destination mailbox address
The mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the "@" sign is invalid. This code is only useful for permanent failures.