Supplementary explanation

Doc/Sdi/mail.xml

@@ -113,9 +113,7 @@
     <title>Exercises</title>
 
     <para>Our aim is to install and configure the <acronym>MTA</acronym>
-    Postfix with many features used in production today.</para>
-
-    <para/>
+    Postfix. Sending, delivering and receiving an email works like:</para>
 
     <informalfigure>
       <mediaobject>