I keep coming up against this, and I keep forgetting it, so I figured I'd write it down here for all of our benefit.
Exchange 2003 allowed us to easily mail enable public folders, so something sent to email@example.com would go to a public folder where any number of staff could monitor the mailbox.
However, by default, the mail is stored in the Public Folder as a NOTE and not an E-MAIL (for the geeks in the audience IPM.POST vs. IPM.NOTE)
To make the public folder store incoming mail as emails, we need to make a quick registry change. This is all outlined in MS KB 817809.
And create (or edit) the key:
Value name: Incoming defaults to IPM.Note
Value type: DWORD
Value data: 1
Setting the value to 1 (true) stores things as IPM.NOTE (which is what we want). Setting the value to 0 sets it back to saving things as a post.