No difference in forward and redirect on Zimbra

We have the case, that we need to forward all incoming emails to a other email address in the same domain.

Here is what is happening with forward:

1. test-a@example.com receives a email from outside@gmail.com
2. test-a@example.com forwards the email to end@example.com
3. The email should show from: test-a@example.com but still shows from: outside@gmail.com

Here is what is happening with redirect:

1. test-a@example.com receives a email from outside@gmail.com
2. test-a@example.com redirects the email to end@example.com
3. The email shows from: outside@gmail.com

So the redirect works as intended, but the forward does not.

Can anyone tell me why Zimbra is not functioning as I am expecting?