How to edit the template of a node teaser that is sent in a newsletter email?
I think it would be easiest to change the teaser in the Manage Display tab under the appropriate content type. That will apply everywhere.

However if you have custom theme stuff you want to accomplish you'll have to make sure you're affecting the correct theme. It sounds like the e-mail generator may be using a different theme to send the e-mail?

A simple way to test for this is putting &lt;h1&gt;HELLO&lt;/h1&gt; in a template that you think it's using.