I'm having the exact same issue (I have an HTC Glacier, rather than an EVO).
When using the native HTC Mail app, connecting to a Yahoo Mail account (among others), certain emails have their body content completely disappear after the initial open. I see the content for a brief second, then the body goes blank (completely white). Header area still shows, and the email still shows up in the listing, but trying to open it again shows just the white content area. Doesn't affect all Yahoo emails, just some.
I've force quit the app, cleared all data and cache... still the same issue.
I've manually edited the account server settings, removing the "android." prepending both the incoming and outgoing server. This seems to have allowed me to view SOME of the emails that were previously blank, but not all.
From what I gather, the cause is the Yahoo Today feature producing some malformed HTML. However, I think both Yahoo AND HTC are at fault. Yes, Yahoo is producing bad HTML, but, obviously, other Android email clients seem to be able to handle this bad HTML just fine. It only seems to be HTC's Mail app that has issues.
I did download the K-9 Mail app, and while it solves the problem, I actually don't much care for the interface. I also have tried the Yahoo Mail app, and it solves the problem as well, but, again, usability is horrendous. I don't consider either of these to be a viable solution.
I do hope HTC fixes the mail app soon.