With Liferay 5.1, we introduced a new Mail Portlet.  Here are instructions on how to add it

Start by getting to the "Plugins Installer".  You'll see that the Mail Portlet is one of the options.

Select the latest version of the Mail Portlet and click "Install".

The Mail Portlet will start to install and you will see a message that it has installed successfully.

Add it to a page

You'll see that your Tomcat console has the Mail Portlet install logs.

Start using it!