If you want to send email from your app, you have 2 options:
You can use an external email service like mandrill/sendinblue/Mailgun etc.
Does services do have an easy API integration available to send emails from your application.
The other option is to use Mendix own ability to send email or use SMTP through the EmailTemplates module of Mendix.
This is a bit more complex, but you don't need an external service for it.