I´m very happy that I discovered Zimbra! I have some domains where I set up the MX record to the Zimbra domain mailhost.xxMYDOMAINxx.de.
Everything works fine except sending mails to any user @web.de. When sending an email to a @web.de-user I immediately get the following error message as an email:

This is the mail system at host mailhost.xxMYDOMAINxx.de.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<USER@web.de>: web.de
This seems to be a strange message, because it does not contain an error description. Do you have a hint what the problem could be?