It definitely wasn't a Coyote Creek migration.
Christ, pay me $10M and I'll bring an army of 30 geeks to turnaround this landfill fire. I did a startup that M&A'ed organization email providers of all sorts (Exchange, postfix, sendmail, etc.) to aggregate them for efficiencies at scale.
1. Tested and complete backups
2. Sample testing
3. mbox migrations
4. Exceptions handling
5. Further testing
6. Cutover