Not sure if this will work, but it's worth a try - here goes...

I assume you're using the HTC email client (as opposed to the Gmail one)?
When you delete an email, it doesn't actually delete it but moves it to the 'Trash' folder. If you've never emptied this, you may be running into some sort file number/size limitation for that folder.

When in your inbox, Menu-> Folders -> Trash; Menu -> Delete; Menu -> Mark All -> Delete

On completion your inbox will also appear empty but I think it just takes a while to sort out it's database after deleting a large quantity of emails - they should reappear after a minute or so.