I search the settings, where I can fix my problem... Actually I can send an eMail to each address, which I want.
If I answer to this eMail, I can't fetch this mail with a client. Reason therefore is - so I think - an blocked receiver list.
What do I have to do, that I can receive eMails from external addresses?
If you want or need some config files, I can post them.
I hope, somebody can help me.