I get the feeling you are unsure about the migration. In which case I strongly urge you to contact Sun Professional Services and inquire about obtaining assistance.I would expect at least two engagements would be needed. One engagement to do an assesment of your current environment and then to write up a high level plan on getting to iMS 5.2. The 2nd phase would be to actually do the migration.Sun also has partners that perform these tasks. Contact your Sun sales representative for more information.
Mail migrations are complex things to do seamlessly with as little interruption to end users as possible. I've had many years of experience doing migrations. Though I admit the last time I touched an MSX machine was '99.I personally dispise MSX as a mail server.