No. The only supported upgrade in place is from Exchange 2000 SP3 or later. You would need to first upgrade your Exchange 5.5 server to at least Exchange 2000 SP3 and then upgrade in place to Exchange 2003. Another option is to exmerge out your current users and exmerge them into an Exchange 2003 server. And the only other option is called the leap frog migration. You configure the Active Directory Connector (ADC) for Exchange 2003 between the Active Directory and Exchange 5.5 Directory Service. Install a new Exchange 2003 server into the enterprise and move the Exchange 5.5 users to Exchange 2003.