To use an SMTP server to send the emails go to Quform → Settings on the WordPress menu. Go to the Email sending settings section and at Email sending method choose SMTP then enter your connection details. For example, to send email via Gmail it may look something like this.
SMTP host: smtp.gmail.com
SMTP port: 465
SMTP encryption: SSL
SMTP username: firstname.lastname@example.org
SMTP password: your-password
Reliable SMTP servers
There are some premium and free SMTP services available which are probably more reliable than your own mail server and have a higher delivery success.
Free (up to 100,000 emails per month)
SparkPost have a free SMTP service that allow you to send 100,000 emails per month for free (10,000 per day). The service also has a Dashboard where you can track each sent email. Visit https://www.sparkpost.com to sign up.
Free (up to 6,000 emails per month)
Mailjet have a free SMTP service that allow you to send 6,000 emails per month for free (200 per day). The service also has a Dashboard where you can track each sent email. Visit https://www.mailjet.com to sign up and see this page for how to get the SMTP settings.
You can use the free Gmail SMTP service to send emails, example settings are at the top of this page.
Troubleshooting the SMTP connection
See the main article Testing email sending.
Using SMTP on a local server (WAMP, XAMPP etc.)
If you are developing your site locally with a web server package such as XAMPP, you will need to enable OpenSSL in PHP to connect to SMTP servers which use TLS or SSL. See this page to find out how.