Hi there
My guess would be the Enter mode on the ck editor
It defaults to Paragraph, as you can see, meaning when you hit “enter” to create a new line, it’s inserting a <P> tag to the markup. As you can see there’s a “shift+enter” option which inserts a linebreak, or <br>.
So try writing your email template using “shift+enter” when you want to insert a line break like this!
(This is assuming the email module uses the default ckeditor settings, I think it does)
Hope this helps!