Exchange 2003 to Exchange 2010 Transition

Hello,

We are planning to do transition from Exchange 2003 to 2010.

I want to know, till what phase of the transition there will be no impact to the end users in terms of mail flow, accessing outlook etc.

For ease, I have also made use of "Exchange 2010 Deployment Assistant" which tells step by step, in my transition plan.

Thanks,

Mihir