Thanks for fast answer. However, it does not work or more explicitely, I don't understand it.
I added a traffic rule...

According to my understanding, this "opens" the port, i.e. if something from wan zone from any port from any ip enteres the device 25 port, it can pass.
Moreover, I added a port forwarding rule...

According to my understanding this is the "rewriting rule", i.e. anything that comes from wan from any ip with any port and enters the device at port 25 (=external port????) is forwarded to lan to the given IP address (redlined for security reasons) at port 25. This port is open, checked it with nmap.
If I perform an external test...

it tells me that the port is closed (the redlined part is of course the static, public IP address of my router given to me by my ISP).
What am I doing wrong here?
Thanks a lot for help.
Regards,
Equilibrium
*** Update ***
I used another port scanner now...

This scanner shows that all my mail ports are in stealth mode. This means Router is O.K.
Problem must be mail server.
Thanks a lot for help.
Regards,
Equilibrium