Thread: java.net.UnknownHostException : Exception while resolving host name via InetAddress.getByName()

I am using "InetAddress.getByName("hostname")" to resolve the host name of smart host before sending the mail through SMTP.

Getting exception while resolving Host : Host Name=smarthost.demo.com Exception=java.net.UnknownHostException: smarthost.demo.com: Name or service not known. This issue only pops up when I send smtp messages back to back, after initial few success messages I am getting "unknown host exception", but if I restart the server it works fine again for inital few messages.

My observation - It's working fine with Java 7 but on Java 8 facing this issue but couldn't find relevant proofs for the same. Is it really because of Java versions or any other possible solution or reason is there ?

When you mention Java7 vs Java8, are you referring to the JDK version used to build the application, or the JRE on the runtime platform?

If you need to reboot the server to fix the problem, then that sounds more like an OS/platform issue rather than Java application issue.
- Are you confident that the platform's DNS resolvers are acting properly?
- If you use a platform utility like nslookup or dig, do you notice any unusual?
- Can you snoop the network traffic to the resolvers using wireshark or tcdump and see what is happening?

If you are looking-up an SMTP smarthost, shouldn't you only really need to do this once (or very infrequently)? Smarthosts typically wouldn't change their IP addresses often.