Go to the log viewer on the Nethserver webinterface, and search for the IP or hostname of your sending mailserver or client. This should at least yield connection info. If not, the firewall is blocking it.
Most firewalls do not automatically log every block … so perhaps you need to turn on logging for a port25 rule.
NAT could be an issue. DNS could be the issue. If your computer uses a buplic, or buplic looking DNS, you will get no result for your maildomain back and your client wont be able to send it.
If your maildomain is known on internet, it will likely be forwarded to your public IP.
The DNS server handling the request should have a valid MX record.
Perhaps your provider is blocking you on there not being a public MX record as this is oftent the case for malicious mailservers.
Check out: How to get rid smarthost (and use rdns,spf,dkim,dmarc) This topic is a wealth of info on how to configure and check your mailserver config.
Does your mailclient throw an error during send/receive ?