I have done half installation of an Exchange 5.5 to Exchange 2003 migration, which needs to be done without any downtime. I have also got Windows NT 4.0 domain with an Exchange 5.5 server istalled and have created a windows 2003 domain and also a 2 way trust relationship between the 2003 domain and nt domain. After that Exchange 2003 was installed into the Exchange 5.5 organisation. I have migrated the email accounts from Exchange 5.5 to Exchange 2003. The users are now authenticating onto the Windows NT 4.0 domain and then accessins their email accounts on the Exchange 2003 sever. The Exchange 5.5 server is removed by me from the organisation as all accounts have been removed. I wanted to know, if I now convert my Windows 2003 domain from mixed to native mod, then will all the users lose connectivity to their email accounts on the Exchange 2003 server since they are authenticating to the NT domain. Thanks for any ideas.