Need help with scenario

I need a help with the transition I have in mind. I want to migrate one email domain from SBS2003 and other one from MailCleaner to UCS. I want to be able to send and receive messages from and to both mail domains. Messages can reside in different mailstores or in one singel - doesn’t matter too much. Users should be able to log in to any email domain using the same username/password.

Right now I’m playing with a 3rd (personal) domain trying to set a UCS mailhost with users being authenticated by their AD username password. It is working somehow, but there are still a couple of troubles.

First of all, any message I am sending from the new server does contain signs from parent domain name, although it is better not to have these signs at all.

Other really BIG issue is the fact that the user list does not contain all the users from the parent domain - there are a couple of users that was not imported/shared in/to the UCS domain management. This is a critical issue as long as I cannot create a mailbox for these missing users. Furthermore, if I migrate the domain from AD to UCS, I will still be missing these users. Where shall I look for the root of this problem?

Any help is highly appreciated, thanks.

Hi All!
Keeping in mind that plenty of answers and recommendations I’ve received in this topic, there is no convenient scenario to move an infrastructure from, let’s say, Windows 2008/2012 to Univention Corporate Server. At least for free.
Ok, I’ll have to stay on on Windows AD with Zimbra, as it is ok more or less for a small company with AD. It’s a pity, - I started to like UCS.
So long!