Lookup for "500 5.1.3"
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 (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (5.1.3)
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.