Thank you awedor for your fast reply.
sadly that option
“Sender based on Reply-To header” under settings → channels → email
has no effect in my case (i don’t know why cause it seems to fit).
I restarted zammad to be sure that the option is active.
Or you have to change the way the e-mails are forwarded. Depending on how you set that up you might have different options how mails are forwarded.
What i can do is setup an IMAP account on my mailserver for the redirect and than it works but that feels kinda redundant.
Or i remove my LDAP connection, but that is even worse 
in my opinion.
I’m open for new suggestions.
Thank you in advance.