You can use the Mail Client from both back and front end. Also it is possible to integrate the component with other extensions.
Actually the component takes advantage the magic Joomla! variable tmpl=component where you can strip the template and just show the component itself. With this ability, you can use for an example an iframe, to show the component as a part of any other extension you might have. You can even add it within an article, given that your editor does not strip the html tags.
The first thing to do is to go to SP Mail Client options, and set option to work with irame as Yes.
Then you must create front end menu. To do that from the back end, menu manager, you just add a new menu item, and select SP Mail Client, as below image.
After that and we assume that you use sef, then just add the iframe any place you wish. The url to add in iframe should be: 'www.yourdomain.com/mails?tmpl=component'. The result should be something like below image!
If for some reason you added the iframe as a unique url, if you press the envelope url in the navigation bar, then you will be redirected to the home page of your site.