Microsoft Exchange Server 2010 : Email delay sending

Dear All,

I've have migrated one of our client email system to on-premise exchange solution.

So far, everything is working fine except email delay sending to one specific domain and i am seeing a lot of SMTP Log message as below

Failed to connect. Error Code: 10060, Error Message: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 122.xxx.120.103:25

Actually, this IP address (122.xxx.120.103) is our email public IP which is configured in firewall and route with NAT

let me know if you have any suggestion on it.

Thanks