Depends on the ROM you're using. On the stock sprint 6.5 ROM...

[HKCU\Software\HTC\HTCSENSOR\GSensor\WhiteList]
Delete the string for "Messaging" with a value of "tmail.exe"

You can also...

[HKCU\Software\HTC\HTCSENSOR\GSensor]
Set AutoRotation = 0

Which will disable autorotation in other programs such as navigation, pIE, and the calculator. Opera's rotation will still work fine.

I believe you will have to soft reset after deleting the messaging entry, but not after changing autorotation to 0.

I'm not 100% on the name of the string, but I know it's value is tmail.exe so just look for that one.