Current setupExchange 5.5 sp4 on NT4 sp6 We recently upgrade our 2000 native mode domain to 2003 r3. After a week of migration our nt4 box got reboot and now exchange 5.5 is not working and the nt4 box is unable to resolve domain accounts name. domain accounts are showing up in with its SID when you view the property of the local group. And outlook client are not excepting the domain password. Any help, lead or info will be appreciated.

You said 2003 R3. I think you meant "R2" but this isn't a domain functional level, so I'll further assume you mean 2003 native mode. Do you have any Nt4 domain controllers left? This obviously would be a problem. Do you have any Exchange 2000, 2003 or 2007 servers in the environment?

Ouch! It sounds like the "trust" between the NT 4.0 system and the domain is broken. Can you logon to your domain account via the the NT Server? Are you still using WINS? Can the NT 4.0 system still communicate with the domain?