HTML Mail lets you theme your outgoing messages the same way you theme the rest of your website.
The following additional modules, while not required, are highly recommended:
Also available as a patch.
Visit the Mail System settings page at admin/settings/mailsystem to select which parts of Drupal will use HTML Mail instead of the default mail system.
Visit the HTML Mail settings page at admin/settings/htmlmail to select a theme, pre-filter, and post-filter for your messages.
The email message text goes through three transformations before sending:
A template file is applied to your message header, subject, and body text. The default template is the included htmlmail.tpl.php file. You may copy this file to your default theme directory and use it to customize the contents and formatting of your messages. The comments within that file contain complete documentation on its usage.
(Optional: requires Echo module.) The templated text may be wrapped in a website theme. This lets you use any one of over 800 themes to style your messages. Creating an email-specific sub-theme lets you use the full power of the drupal theme system to format your messages.
You may choose a text format to be used for filtering email messages after theming. This allows you to use any combination of over 200 filter modules to make final changes to your message before sending.
Here is a recommended configuration:
Emogrifier Converts stylesheets to inline style rules for consistent display on mobile devices and webmail.
Transliteration Converts non-ASCII text to US-ASCII equivalents. This helps prevent Microsoft "smart-quotes" from appearing as question-marks in Mozilla Thunderbird.
Pathologic Converts relative URLS to absolute URLS so that clickable links in your message will work as intended.
Visit the issue queue for support and feature requests.
http://drupal.org/project/emogrifier
http://drupal.org/project/htmlpurifier
http://drupal.org/project/htmlawed
http://drupal.org/project/mailmime
http://drupal.org/project/mailsystem
http://drupal.org/project/pathologic
http://drupal.org/project/transliteration
D7: http://api.drupal.org/api/drupal/modules--filter--filter.module/7
D7: http://drupal.org/documentation/install/modules-themes/modules-7
http://drupal.org/documentation/theme