You can probably set up something like this:
If your main domain is domain.com, configure the scalix server with name scalix.domain.com. Then create all users under scalix to be like
userX@scalix.domain.com. Then forward all mail for
userX@domain.com to
userX@scalix.domain.com. Don't forget to add a forward and reverse DNS entry for scalix.domain.com with the public IP of the scalix server in your domain.com DNS file at the ISP or whoever manages your DNS. That should do the trick but it'll require you to change everyone's mail client to receive mail for
userX@scalix.domain.com instead of
user@domain.com. For sending out I'm not sure about your setup but I think the easiest will be to send through the scalix server and configure sendmail to masquerade all users as
userX@domain.com instead of
userX@scalix.domain.com.
Alternatively, setup your scalix server as for domain.com, remove the accounts at the ISP and have the ISP only to queue emails for you in case server is broken or no connectivity.