Lookup for "503 2.1.2"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (503)
Bad sequence of commands
Enhanced Status Code Details (2.1.2)
Bad destination system address
The destination system specified in the address does not exist or is incapable of accepting mail. For Internet mail names, this means the address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures.