Hi all! I ve solved my problem!

Here is my solution:
To activate the output in /var/log/zimbra.log
Code:
postconf -e debug_peer_list=mail.host.com
postconf -e debug_peer_level=3
postfix reload
and then I restart the whole server. I guess normaly it should not be neccessary, but in my case it was.
The best site was the wiki site to the
Outgoing SMTP Authentication. I used the most options/commands.
The message "553 sorry, that domain isn't in my list of allowed rcpthosts; no valid cert for gatewaying" seems to be like the call of a baby "wäääh" and its up to you to figure out what its missing
In my case it was a lot:
- misspelled the username for the smtp-auth
- the SMTP Port should not be 25 but 587
- then set smtp_sasl_auth_enable=yes
- I ve to remove the noplaintextoption (smtp_sasl_security_options=noanonymous)
And now I can send emails via my relayserver

Tim