No firewall has been set yet. So is there any way to find what is the problem?
Ask your network administrator. 
Your test indicates outbound connections from your host system aren’t functioning. That’s outside the scope of Nextcloud itself. It’s an OS / host / network infrastructure matter.
You might see if you can make outbound connections to anything from your server (or if it’s more limited to your SMTP server).
Either way, we have no idea since it’s not something Nextcloud has any control over.
Also, keep in mind that since you didn’t fill out the support template we have even less idea about your server / hosting environment. 