If you need a free and secure SMTP you can use your Gmail SMTP to send emails from your app. You just have to enable a few things to make it work. This blog post shows you what are the settings you need to enable to make it work.

1. First, go to your Gmail Settings > Forwarding and POP/IMAP 

Enable both POP and IMAP.

2. Next is to change your Google account settings and Allow less secured app to use gmail SMTP

This just means that you are allowing 3rd party applications to use your SMTP. You can still secure this by using username and password on your SMTP settings.

Go to this link https://myaccount.google.com/lesssecureapps or https://myaccount.google.com/security

Now your Gmail SMTP is setup and ready to use!

Depending on your application you can now add it to your SMTP settings. In ASP.Net Web App, we need to add it in our web.config.