Lookup for "500 4.1.3"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (4.1.3)
Bad destination mailbox address syntax
The destination address was syntactically invalid. This can apply to any field in the address. This code is only useful for permanent failures.