I tried to set up a front-end server, but my IMAP and Outlook Web Access (OWA) users can't get to their mailboxes on the front-end box. What's wrong?

This behavior is a feature, not a flaw. The purpose of front-end servers is to proxy Internet protocol requests to a back-end server. When you put mailboxes on a front-end server, you might be placing mailbox data outside your security perimeter, and you're definitely commingling the proper function of a front-end server—proxy-ing—with that of a mailbox server, which most people think is a bad idea. Because the front-end server can't proxy to itself, your Internet protocol users can't get their mail. (If they used Messaging API—MAPI—clients, they'd be OK, because the front-end server can't proxy MAPI requests.) To solve the problem, move your users' mailboxes to the back-end server. Bear in mind that the regular mailbox move process uses MAPI, so you might have to rely on the trusty old Exmerge utility instead.