The default paragraph style has some margin.

- You can do Shift+Enter to get a `br` without breaking the paragraph. - You can change the format from "Paragraph" to "Body Text" to remove the margin. Note that Thunderbird changes new lines back to "Paragraph" automatically, so you need to frist write your email, then format it as "Body Text". - Or, you can disable the "Use Paragraph format instead of Body text by default" option in the settings, to always have "Body text".

Good to know.

I've always wondered why HTML editors tend to work this way (Wordpress is the same), instead of having a single enter key be a line break and a double enter key be a paragraph.