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.
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'.