Postfix rewrittes alias

Hi,

I installed Kopano on Debian 9 as described in the Kopano Core Administrator Manual with Postfix and OpenLDAP. Everything seems to be working fine, except aliases. I created aliases for several users in LDAP and when email is sent to those aliases the to field gets rewritten by Postfix. So an email sent to alias1@example.com is correctly delivered to user1@example.com, but the to field in the user’s inbox shows user1@example.com and not alias1@example.com. Is there any way to change this behavior so that the to field gets preserved for aliases?