There isn't a specific way to do this in smtpd.cfg but there will be in the next release due Q1 06. As a side note, all the other daemons such as POP, IMAP and LDAP will listen on the wildcard, i.e. 0.0.0.0 address which is effectively all unused interfaces.
However, you can make use of a setting in general.cfg.
You can specify the interface that all listening daemons will listen on by adding the line:
where hostname is the name of the interface you want them to bind to.
You will need to restart Scalix
Please note that this setting will not be available in the next release as we're introducing support for multiple instances in a high availablity scenario which removes the need for this.
Cheers
Dave