UCS + Kopano + t-online.de => searching best practise

Hi everybody,
I am in search for a best practise to set up UCS + Kopano + t-online.de.
We have a UCS4.4-8 Errata 1060 with Kopano Core 8.7.1.0-1 and Kopano WebApp 3.5.14.2539-2 running. There are two users for whom we’d like to fetch Email from t-online.de using pop3.
Can anybody give us a best practise for that scenario? I couldn’t find anything online.

My first approach was to apply internal dummy emailadresses (e.g. user1@domain.local) to each user. Therefore we used fetchmail with pop3 and managed to poll the mails from t-online and deliver them to the according Kopano mailboxes. Outgoing we configured the smtp-relay securesmtprelay.t-online.de. We were able to send outgoing mails, but the mail contained the internal dummy mailadresss as the senders address.

The second approach was to use the actual external t-online emailadresses for each user. Sending the mails via the smtp-relay worked fine, but polling the mails from t-online via pop3 wasn’t. Postfix realized that the maildomain t-online.de is an external one and forwarded the mails right back to the smtp-relay. A perfect loop.

The third idea we had was, to poll the mails via fetchmail and pop3 and forward them to kopano-dagent with the mda option. The result is, that fetchmail polled the mails, but kopano-dagent answered with kind of an auth error (NULL password).

I hope someone can help and point out a rough direction how to configure UCS+Kopano+t-online.

Many thanks in advance!
Ingo