What are the differences between Exchange Server 5.5 and Exchange 2000?
Do you want the whole list, or just the top 100? Seriously, Exchange 2000 is a very different product.
The primary differences are...
-Exchange 2000 does not have its own directory or directory service; it uses Active Directory instead.
-Exchange 2000 uses native components of Windows 2000 (namely, IIS and its SMTP, NNTP, W3SVC and other components, Kerberos and others) for many core functions.
-SMTP is now a full peer to RPC, and is it the default transport protocol between Exchange 2000 servers.
-Exchange 2000 supports Active/Active clustering and was recently certified for Windows 2000 Datacenter.
-Exchange 2000 scales much higher.
-It boasts conferencing services and instant messaging.
There's way too many more to mention here. Check out https://products.office.com/en-US/exchange for more details.