Domino smarthost and "Smart host is used for all local recipients"

Good afternoon,

Is it possible to apply the fact of perform the "Smart host is used for all recipients" except if mail comes from this ip which is basically the own smarthost?.

I'm trying to have two mail servers. One of the Domino system. Another one a Unix system. Some users will end up in the Unix system definitively. Others will remain in Domino. I don't want to remove the users from Domino until perhaps one year from now, but wanted the mail from them to be delivered to a smarthost (even when the account exists in Domino, because now they use the unix account), who knows where to deliver the email.

Is this possible?. I mean, everything to the smarthost, except if it is delivering the mail to us, because it has decided it's for Domino...

Can this be done?. Perhaps using different router instances or....

Cheers,