Mime Mail lets you sent HTML email from Drupal plus Mime Mail automatically creates the plain text version.

Download Mime Mail from drupal.org/project/mimemail. Install and configure Mime Mail first then install modules that use Mime Mail. Simplenews uses Mime Mail.

Configuration

Select Administer » Site configuration » Mime Mail

Set the default e-mail address used as the from address. Applications can provide their own email address when they send mail to mime Mail.

You can set Mime Mail to process all outgoing email. With this option, system emails will have styles and formatting

Mime mail can filter mail and the default filter is set to one of your content filter types including Filtered HTML, Full HTML, or PHP code.

You can force plain text email only but then you would not need Mime Mail.

There is an option to process incoming messages posted to your site and an option to set and use a message validation string.