The Thunderbird Stationary extension lets you edit the message HTML.
I use Mozilla Thunderbird as my primary e-mail client and before you say it – I know everyone uses web mail these days – that is if they even use e-mail. However I have almost twenty inboxes to monitor, across a variety of hosts, and it is just easier to use Thunderbird to download them all and only link the important ones to the iPhone. One of the things that frustrated me about Thunderbird was that I could not edit the message HTML. I could see it in the Message Source view, but there was no way to change it. Then I found the Stationary extension.
This extension is primarily to add message templates (similar to MS Outlook), however it also includes a feature that adds a tab to the message editing panel to switch to a HTML editor. So now I can tweak the message HTML anyway I please.