Index: mimemail.module
===================================================================
--- mimemail.module	(revision 517)
+++ mimemail.module	(working copy)
@@ -387,7 +387,12 @@ function theme_mimemail_message($body, $
 
   // attempt to include a mail-specific version of the css.
   // if you want smaller mail messages, add a mail.css file to your theme
-  $styles = path_to_theme() .'/mail.css';
+  if (function_exists('path_to_subtheme')) { // attempt to use zen subtheme first
+    $styles = path_to_subtheme() .'/mail.css';
+  }
+  if (!function_exists('path_to_subtheme') or !file_exists($styles)) { // no zen or no subtheme mail.css file
+    $styles = path_to_theme() .'/mail.css';
+  }
 
   $output .= '<style type="text/css"><!--';
   if (!file_exists($styles)) {