By default Scalix configures itself to operate as a standalone mail server which is normally what client's want. Unfortunately the IP range my ISP owns is frequently flagged by anti-spam services making mail delivery to certain recipients challenging. Fortunately my ISP provides an SMTP relay server for customers on the network which is not effected by the broad blocks applied to the IP range I am on.
To configure Scalix to use the SMTP relay for outgoing mail edit /etc/mail/sendmail.cf and find the line that reads:
DS
And change it to:
DSrelay.smtpserver.com
Restart sendmail and you will find all your external mail sent via Scalix is relayed via the ISP's SMTP server. To check it is all working send an email and then view the /var/logs/mail file, the last entry should show a log of the outgoing mail with the relay recorded next to it.
If you make this change just make sure your domain SPF record is modified to ensure it defines the ISP's SMTP server as a source that is allowed to send mail on behalf of your name. The best SPF wizard I have found is hosted by OpenSPF and makes creating this potentially complicated DNS entry straightforward.