Sitecore: Sending mails with local MTA

When you working with Sitecore EXM you obviously need to test sending mails (review) creted using EXM.
Sometimes you might get the following error in a error strip when try to send email from EXM.

Your Sitecore App Center credentials are incorrect, or you do not have a valid license. Correct your credentials in <a href='/sitecore/shell/Applications/SitecoreApps/Loader.aspx' target='_blank'>Sitecore App Center</a> or contact Sitecore Customer Service.

This is because Sitecore EXM is using the Sitecore AppCenter's Mail Transfer Agent (MTA).

If you are using the AppCenter MTA (Mail Transfer Agent), then you can sign into the AppCenter and let the AppCenter MTA to do the job for you.

In my case I don't won't this. I need to use my own MTA.
To do this you have to do the following:
1. Set the UseLocalMTA value to 'true' (default is false).
2. Set your own SMTP settings.

Q: Where is this 'UseLocalMTA' setting is ? How can I find it ?
A: Simple. Get a configurations of your website using this URL: 
http://<your_host_name>/sitecore/admin/showconfig.aspx
Then Search for 'UseLocalMTA'.
It will find you the following configuration.
<setting name="UseLocalMTA" value="false" patch:source="Sitecore.EmailExperience.ContentManagement.config"/>

Go to the patch.source using the file system of your website.
Then change the UseLocalMTA value to 'true'.