Sitecore ECM QuickStart

Using Local MTA vs Sitecore AppCenter

If you are using your own SMTP Server to send emails, you will need to adjust this setting to true in Sitecore.EmailCampaign.config

      <!--Set it to "true" if you want use the SMTP settings below. You should purchase the right of using the "UseLocalMTA" setting first.-->
      <setting name="UseLocalMTA" value="false" />

Reference: Local-Mail-Servers-and Sitecore