Windows NT accounts have a 20-character limit. If you have user names that are greater than the 20-character limit and you plan to create Windows NT accounts as part of mailbox creation, you must use the two-step migration method and modify the primary files before importing them. The Assoc-NT-Account field must be edited to be unique and must contain no more than 20 characters. You can use the auto naming feature in the Microsoft Exchange Server Administrator program to change the alias form and force all Windows NT user accounts to be less than 20 characters, regardless of users' actual names. For more information about the auto naming feature, see Microsoft Exchange Server Getting Started.
If you do not do this, the Migration Wizard generates an error message for each attempt to create the Windows NT account, including one per mail message it tries to import. The data that is not imported is written to the recovery file, which can be edited and imported. For more information on recovery files, see Appendix B.