Meanwhile the service hotline from my ISP told me, that a certificate is needed to send emails. Well, I don't know what kind of certificate, where to get it, and how to implement it. IMHO it should be a certificate of my ISP because as already reported, when sending email with a email client out of my network, I had to accept a certificate from my ISP. After accepting the cert sending emails was no problem.
Well anyway, there should come up a solution very soon. I think most people using a Small Enterprise Server do not have a static IP, and use their ISP as relay. Imagine the situation in a small company not beeing able to send emails!
Bug 8206 submitted.
Followed the bugtracker. Latest updates installed, thank's to Charlie.