Lookup for "500 2.1.1"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (2.1.1)
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.