Bullets, like most things with HTML emails, are not simple if you want to deviate from the standard. Newer versions of Outlook doesn't like list-style-image or list-style-type in the CSS.
The best workaround that I have found is simply using paragraph (<p>) tags to list my items, and preface them with the image or symbol I want as the actual bullet. <p><img src="mybullet.gif" alt="•"> My first item</p>
You can try playing around with the CSS and see if you can get it to work, but make sure to run it through a rendering system to ensure it works properly across all clients.