External links added.

fix #565

doc/getting_started/configuration.rst

@@ -66,6 +66,14 @@ The *admin* application exposes several parameters, they are presented below:
 |                    |is specified        |                    |
 +--------------------+--------------------+--------------------+
 
+.. note::
+
+   If you are not familiar with virtual domain hosting, you should
+   take a look at `postfix's documentation
+   <http://www.postfix.org/VIRTUAL_README.html>`_. This `How to
+   <https://help.ubuntu.com/community/PostfixVirtualMailBoxClamSmtpHowto>`_
+   also contains useful information.
+
 .. note::
 
    A random secret key will be generated each time the *Parameters*

doc/getting_started/install.rst

@@ -155,8 +155,9 @@ server::
 
 You should be able to access Modoboa at http://localhost:8000/.
 
-For a fully working interface using the embedded HTTP server, you need to set the ``DEBUG``
-parameter in settings.py to ``True``.
+For a fully working interface using the embedded HTTP server, you need
+to set the ``DEBUG`` parameter in settings.py to ``True``.
 
 For a production environment, we recommend using a stable webserver
-like :ref:`apache2` or :ref:`nginx-label`. Don't forget to set ``DEBUG`` back to ``False``.
+like :ref:`apache2` or :ref:`nginx-label`. Don't forget to set
+``DEBUG`` back to ``False``.