/Channels/Exchange/ChannelHelper.cs has some code in it that basically says that if a URL pasted into the configuration doesn't end with .asmx, then append /Ews/Exchange.asmx
However, only Exchange 2007 and 2010 use the /Ews/Exchange.asmx URL's. Exchange 2003 uses /exchange
sigh I know, it's old, but lots of people still are stuck with it in their enterprise.