click settings>composition and UNtick paragraph format.

This is how I fixed it.

Go to Tools > Settings, then scroll all the way to the bottom and click on Config Editor. Type paragraph in the search line. You will see a line = mail.compose.default_to_paragraph This is NOT what you want, so, over to the right click on the two arrows (top/bottom) to turn this to False Now it will work for you.

I recommend the settings>composition setting instead, as it does the same thing without users needing to use config editor.