Can you check /etc/opt/scalix/webmail/partner.xml and ensure that the smtpServer value is correct. You will need to restart Tomcat if you have to make any changes.
If this is OK, check /var/opt/scalix/sys/smtpd.cfg and the RELAY accept lines there. If you just have a .domain.com entry but you have not got reverse DNS configured, submitting mail to the outside world will be prevented.
In that case, you need to add a line, above the RELAY reject entry, that includes the IP address of your server.
You will need to restart the SMTP Relay with
Cheers
Dave