My email provider's SMTP server has been rejecting some of my emails as spam and after talking to the admins they said I could reduce the chance of this happening by changing my emails from "HTML only" to "HTML with alternative plain text part". After then reading about such things on the Web that is also a recommendation I found there. So, from reading the Help item in TheBat! it appears that simply changing this setting in my editor preferences will automatically create both an HTML part and a plain text part for each email I send, and the receiver's email client will decide which to display. If that is correct, does the plain text part simply omit anything that can't be displayed as plain text? Are all of the spaces, indenting, and blank lines from the HTML version preserved in the text version?
My suggestion is to compose an email that represents the types of messages you send in HTML - bold, italics, bullets, whatever and then send to yourself. Seriously, that is the best way to answer your question. I regularly do that when I have a question on how a message might appear. That is, I could give a general response that plain text omits any special formatting, such as bold, italics, etc., but it's better to see for yourself.
And if your emails rarely to never use special formatting, then I recommend just using plain text. One of the aspects of TB! that I do not like is that HTML always forces a font, yet it may be one the recipient doesn't like - and that detracts from whatever message you are sending. I respect you didn't ask that, and maybe it's just an annoyance to me. And if there is a way in TB! to send HTML without a font selected, I haven't found it. david