Lookup for "550 4.7.3"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (4.7.3)
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.