Technically, PEWA is not supported by Microsoft PSS, as it is a Resource Kit tool. PSS engineers will give you their "best effort" if you encounter problems with this tool.
After looking into this, I've found that PEWA should work on Exchange 2003. I do not know if Microsoft has tested against Exchange 2003, but since the tool really just does a MAPI logon, and since that function has been relatively unchanged since Exchange 4.0, you should be able to use it without any problems.
One last word of caution, though. I happened to be the co-author of the Exchange 2003 Resource Kit (which is currently under development). PEWA is not on the list of tools to include so it will likely not be included in the Exchange 2003 Resource Kit.
This was first published in April 2004