Hello haroungh,
I would definitely recommend reviewing Ken's comments to start off with. Locate this information on the ESA(s) which are failing to send to these domains and please log into the CLI if you do have access to it and attempt a telnet to that listed host IP.
If that fails then you have some information into why deliveries are failing.
Additionally to allow easier diagnostic if you have the message tracking logs (feel free to edit out any sensitive information such as sender/recipient user names if needed) please do provide.
The hard bounce response would also really help to isolate if it was a network problem, a recipient problem or an email problem.
If the telnet fails; also try (with CLI) nslookup domain.com mx <- see if you get an MX record result; if you do. Is this record the same as the IP your ESA has been trying to deliver to? If not - that indicates you have a host override somewhere in your configuration (content/message filter or SMTP route). If you are meant to have an override (because you own that domain) then it's fine. If it's an external domain out on internet, you may want to look at correcting this.
Let us know how it goes,
Cheers.
Mathew
