There are a couple of problems here. First, the Text property of a textbox does not include any formatting. Second, the .body attribute of an email item does not include any formatting; it is for plain text. You need to copy the textbox text, and paste it into .HTMLbody. HTML requires formatting tags, so you can't just assign the text, you have to paste it and let the application do the conversion for you.
You need to add something like this but I have not tested it. You will also need to set a reference to Microsoft Word.
If you get really stuck let me know and I'll try to find the time to develop an example.
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