Lookup for "500 2.7.3"
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 (2.7.3)
Security conversion required but not possible
A conversion from one secure messaging protocol to another was required for delivery and such conversion was not possible. This is useful only as a permanent error.