The rollout support stuff doesn't expect or try to accomodate HTML mails. It pastes the original message body inside a warning message ("The following email would have been sent..."). See /packages/acs-tcl/tcl/rollout-email-procs.tcl.
So I guess I wouldn't be surprised if this was broken for some mail readers. In Thunderbird, the HTML version, which doesn't include the warning, is displayed. But when I look at the message source, the warning text is there.
It's odd that the rollout support warning doesn't show up in your message source.
