Lookup for "550 4.6.3"
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 (4.6.3)
Conversion required but not supported
The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path. This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop.