Do you use Word, or another word processor? Do you never have similar problems there? Editing text with markup is a bit of an art; there are invisible tags which set text attributes and you need to use certain tricks to be sure to get your cursor inside a formatted zone to be able to extend it. Word is particularly bad in that it is impossible to display the markup. I often work with the setting that shows paragraphs and and spaces etc and this helps me guess where to put the cursor. If I want to insert a paragraph using the same formatting as an existing paragraph, I'll often place the cursor inside the last word of the existing paragraph and insert the carriage return, work on the new paragraph and then repair the word I inserted into. I do this a lot in Word and it works in Thunderbird, too. Additionally, in Thunderbird you can get to see the markup and use that to help navigate. The nic-nac-project.org site has an html viewer and the Stationery addon provides one too. There is also an addon named Extra Formatting Buttons which may help with managing type faces and sizes. And anyway, you can't be 100% confident that your correspondent well see our permit your markup, rendering all your efforts a waste of time.