Make sure to setup the SMTP settings for the workspace/server using MMC

It´s rigth, I checked it before. Any other suggestions?

Do you use a proxy server  ???????????

My email  server needs authentification. Perhaps that is the problem.
Do you know if SPS allows to configure this option?

That will definitely be an issue, it doesn't look like SPS allows to
configure username/password for SMTP server. I will suggest installing SMTP
service on the SPS machine and use the local SMTP instead of your email
server.

SMTP service is pretty straightforward to install and is installed when you
install IIS.