Lookup for "500 5.1.5"
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 (5.1.5)
Destination address valid
This mailbox address as specified was valid. This status code should be used for positive delivery reports.