What do you use to read and send mails with the actual server?
Webmail or a client like Outlook or Thunderbird?
If you use Outlook or Thunderbird check their smtp configurastion and, if it doesn't point to the actual mail server, put it in the smarthost of Nethserver.(check for user and password too!).
Another thing to try to check if the connection works:
Open a shell in your Nethserver and digit:
telnet aspmx.l.google.com 25
You should have a answer similar to:
Connected to aspmx.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP a129si1660374wma.19 - gsmtp
If you have a different answer, post it here.
If you don't have any answer I think that port 25 is closed somewhere (pfsense, ISP, ...)