Also, my config is:
$rcmail_config['prefer_html'] = TRUE;
I have found that if I change that to FALSE, email content shows up more consistently, but most have an "HTML" link that pops open a window with an ugly inline display box.
I guess the behaviour I was shooting for was to always display as HTML, but in the case that the email didn't seem right, you could reload the message in the preview pane as plain text instead of HTML, showing you the full email message.
That just seemed 'normal' operation to me.