To the person who found my blog using the search term ‘iphone exchange “missing emails”‘, it’s an easy answer:

When the iPhone’s Mail.app is setup to use Exchange, it has an initial limit to the number of messages it will grab from the server — the default is the 50 most recent messages. This can be increased to 200.

The key word here is recent – by default, the first 50 recent messages you load will be displayed. Want to see anything other than that? Sorry, you gotta use Outlook Web Access. That’s just how it was designed.