Some mail server settings and data may not be automatically preserved when migrating or upgrading Mac OS X Server v10.4 and v10.5 to Mac OS X Server v10.6. In order to preserve or re-create these settings, read the instructions in this article prior to upgrading or migrating your Mail server. Important: Follow the instructions in the Upgrade and Migration administration guide.
Source:Mac OS X Server v10.6: Mail Server upgrade and migration
