Thank you for the detail about the issue. To make sure I am understanding correctly... in Thunderbird, the email body shows as blank. However, when you view source, you see that there is content there. Is that correct?
I'll have to test more with Thunderbird. These used to work. I'm very unclear when/why it doesn't display for some people. It would be really sad if I had to switch it all back to pure text. The HTML is simple and easier for most people to understand what to do; simple buttons as opposed to displayed URL links.
Unfortunately, I do not have a means to have select users assigned HTML vs. text. I had someone else recently ask this, and I need to see if perhaps there is a plugin that could be used. I generally try to minimize the amount of plugins used, as they make upgrading the forum software much more difficult (plugins for phpbb are actually code modifications, as opposed to true "plugins").
When you say that what's "different" is that they are sent in US-ASCII, what do you normally see for that?
I'm not sure where that is being set, but it may be something I could address. I'm just confused as to why it's so inconsistent. In theory, the text version of the email is supposed to be what gets displayed if the email program doesn't want to show the HTML. My guess is that they are trying to display the HTML, but failing to do so as intended.