Block all incoming and outgoing internet connection, except for one

We have a SUSE server running, which has internet connection.
This is in order to send email alerts.
What I need to do is to block all incoming and outgoing internet traffic, except allowing for emails to be send.
In SUSE firewall you have some nice options, but in my case the internet connection and lan connection are using the same network card.
So I can’t configure internal and external zones.
I don’t want to block anything regarding the internal network.

Is there a simple solution on how to achieve this?

What do you mean with “SUSE server”?
When this openSUSE, then please explain which version.
When this is SLES, then please go to https://forums.suse.com/forum.php (and do not forget to mention there which version of SLES you use).

Sorry Henk, its SLES
will go to the SLES forum
thx