The tables, colors and fonts of the default logwatch html mails looks terrible in Mail.app. Here’s how to patch your `/usr/share/logwatch/default.conf/html/header.html`:

--- header.html    2013-06-30 14:45:00.000000000 +0200
+++ header.html 2013-06-30 15:00:00.000000000 +0200
@@ -5,10 +5,12 @@
 <meta name="generator" content="Logwatch  $Version ( $VDate )">
 <style type="text/css">
   h1 {color: gray; border-bottom: 3px double silver; font-family: sans-serif; }
-  h2 {color: white; border-bottom: 1px solid silver; font-family: sans-serif; }
-  h3 {color: white; border-bottom: 1px solid silver; font-family: sans-serif; }
-  th {background: #6D88AD; text-align: left; font-family: sans-serif; }
-  td {background: #EFEFEF; text-align: left; font-family: courier,serif; font-size: 10px; }
+  h2 {border-bottom: 1px solid silver; font-family: sans-serif; }
+  h3 {border-bottom: 1px solid silver; font-family: sans-serif; }
+  table {border-collapse: collapse; }
+  table,th, td {border: 1px solid white; }
+  th {background: #F8F8F8; text-align: left; font-family: sans-serif; }
+  td {background: #F8F8F8; text-align: left; font-family: courier,serif; }
   li { font-family: sans-serif; }
   .ref {padding-left: 1%; }
   .service {padding-left: 1%; }

Much better!