EXPLANATION:
We try to resolve the reverse name (PTR) of the sending machine's IP address and then try to resolve that name back to it's IP address.
These two must exist and be the same. You get this error if they weren't, or they did not exist in the DNS, when we tried to resolve them.For example currently it seems that your IP address has the reverse name of wellborn.co.kr and that name resolves to 121.78.48.135.
This looks bad, because your IP address does not match the one, which we got from the DNS:
121.78.48.135 is not the same as your connecting IP: 116.193.90.73