MS Mail (AppleTalk) custom forms are based on HyperCard and are not compatible with Microsoft Exchange Server custom forms. Messages created in custom forms are migrated as text so that no data is lost.
If you have many messages that use a particular custom form, you could preserve that information as a form instead of as text. To do so, you must extract it before migration. Use the MS Mail SDK for HyperCard (on the Extras disk of MS Mail version 3.x) to create a HyperCard application for retrieving the information and storing it in a database.
If you are using Microsoft Exchange Server computers as a backbone between MS Mail sites, be aware that custom forms are converted to text when sent across the MS Mail Connector. This could impact coexistence during a multiphase migration.