One of our users brought this to our attention. In IE, if you have a message with particularly long lines and enough body text lines, IE will not show the vertical scroll bar, and will additionally not show the full body text.
This is relatively easy to replicate. Send yourself a message with at least one line that's very wide (132 '=' characters with no spaces, so it won't word-wrap, will do nicely.) Be sure the message is at least 10-20 lines long (or, just long enough that you'd need to scroll vertically when viewing the message.
In IE 7 and IE 6 on WinXP, If the browser window is wide enough to display the entire width of the message, scrolling works normally. If not, you'll be able to partially scroll, but won't see the whole message. If it's narrow enough, you won't even get a vertical scroll bar.
I have not been able to test on Vista, but Firefox on all platforms and Safari on Mac do not exhibit this problem.