There is a very narrow band of upgrade paths to Exchange 2007 due to the complexity of the changes made to pretty much everything. So in order to go from MS Mail to Exchange 2007 you would need to migrate to an intermediate platform (5.5 to 2003 to 2007, or 200x to 2007) for a "migration" or do a wholesale cut over to a new Exchange 2007 org by exporting mail from the old system and reimporting it into Exchange.
For a full list of supported scenarios check this link out:
http://www.microsoft.com/technet/prodtechnol/exchange/E2k7Help/a313c016-0e51-466e-a3de-953e1e0d347d.mspx?mfr=true
Table 1 Supported transition and migration scenarios
Existing messaging system
In-place server upgrade to Exchange 2007
Transition to an Exchange 2007 organization
Migrate to Exchange Server 2007
Exchange Server 5.5
Not supported
Supported, by migrating first to Exchange 2000 Server or Exchange Server 2003, then transitioning Exchange 2000 Server or Exchange Server 2003 to Exchange Server 2007
Exchange 2000 Server
Supported
Exchange Server 2003
Mixed Exchange 2000 Server and Exchange Server 2003 organization
Lotus Notes
Supported, using RedBull tools
Other non-Exchange messaging systems
Supported, using ?? tools
- Joel