If you want no external css file send along you can add an empty mail.css file in your theme folder or override the theme function theme_mimemail_message() that adds the css file, for more information see http://drupal.org/node/291762
Using MimeMail (6.x-1.x-dev) with Simplenews (version 6.x-1.0-rc6), with html turned on, some people get the message with a ton of css code in it. It's odd because we tested this and didn't see any code. But we received several messages from people who received code in the body of the message. Some spam filters blocked the message because of all the code. I posted on MimeMail issues but with no response yet.
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