does this make sense?
No, not really.
For mail to get to the Scalix server, DNS would have been configured, or an MTA on another machine would have been configured, to point domain.com to your Scalix server.
If the mail goes from Scalix and is addressed to domain.com, there is no way to configure sendmail such that if an address exists in Scalix, it is delivered to another machine.
The situation your describing is one that our customers experience in a coexistence scenario. They will typically configure their legacy server to still receive mail on domain.com but then forward that mail to scalix.domain.com. Each user on the Scalix server will have 2 internet addresses,
user@domain.com and
user@scalix.domain.com. This allows them to send as
user@domain.com but receive as
user@scalix.domain.com.
When the period of coexistance is over, the DNS records are modified to point domain.com to the Scalix server and the scalix.domain.com internet addresses are removed.
Cheers
Dave