Without a valid registration number, the zarafa outlook connectivity is limited to 3 users. This is enough for home use, but it can be a problem that you can't connect using outlook because zarafa thinks that more users are active. To reset this do this as root:
cd /var/lib/zarafa mv userdb userdb.off /etc/init.d/zarafa-licensed restart
Now you will have 3 outlook clients free again.
Note: If you have windows machines that already tried and failed to connect, restarting them may help!