When composing messages in OS X Mail, you have the option to use the program's built-in list tool to keep numbered or bulleted lists properly formatted, but you may find its formatting options to be a bit limited.
The list generator is not intended to be a complex option; however, by default it only displays one list entry per paragraph, so if you attempt to create a line break to separate list items or add multiple paragraphs per item, the default behavior is to simply append new list entries.
This can be a bit annoying to deal with, and because it results in condensed lists, it may cause you to avoid using this feature. However, OS X Mail does support options to add line breaks to list items, allowing you to generate multiple paragraphs and other separations in lists.
To do this, simply hold the Control key when pressing Enter or Return, and the program will maintain the list structure and give you a new line to work with. This feature is similar to those in other programs such as Apple's Pages and TextEdit editors, so if you have looked for ways to add line breaks in these programs, then you can do so there as well, without using inspectors or other adjustment panels.