there no migration between Exchange based systems and Domino. Most of this has to do with the inablity to create/dynamically bind to users in the domino directory like we can in Ex55 and AD(2000/2003) - there's also no message migration tools (like ExMerge) or the like.
We have DiRT for Domino but it only works for backing up and restoring Domino based Unity servers of the same version (and it doesn't include messages).