An Exchange mailbox may have multiple SMTP addresses. For example, John Doe has a mailbox with firstname.lastname@example.org primary SMTP address and email@example.com secondary smtp address. As Outlook always resolves both SMTP addresses to mailbox name, there is no way to see what address the message was originally sent to. The only known workaround is looking through message envelope headers (open a message > View > Options...). Suppose John Doe would like to set a rule to put any emails sent to firstname.lastname@example.org to a separate "JohnD-Inbox" folder. When he tries to set the rule, it does not work because of address resolving.
WhichAddress for Exchange 2000/2003 application allows user to see which address the message was originally sent to and set up a rule to separate the messages being sent to different SMTP addresses.
The application is an Exchange store event sink.