diff --git a/app.yaml.sample b/app.yaml.sample
index 0b71cbe..3c5533a 100644
--- a/app.yaml.sample
+++ b/app.yaml.sample
@@ -13,6 +13,7 @@ env_variables:
   TEMPLATE_CONTACT_REQUEST_REPLY: templates/template_reply_contact_request.html
   SMTP_SERVER_ADDR: "smtp.mailgun.org:587"
   SMTP_VERITY_CERT: true
+  SMTP_AUTHENTICATION_ENABLED: true
   SMTP_CLIENT_USERNAME: "postmaster@example.com"
   SMTP_CLIENT_PASSWORD: "postmasterSecretPassWord"
   CONTACT_REPLY_EMAIL: "noreply@example.com"
diff --git a/docs/configuration-variables.md b/docs/configuration-variables.md
index e242ef5..53acf33 100644
--- a/docs/configuration-variables.md
+++ b/docs/configuration-variables.md
@@ -4,6 +4,7 @@ Regardless of the deployment platform (Google App Engine, Kubernetes, Docker), t
 
 * `SMTP_SERVER_ADDR`: Set the address of the SMTP server in the form of `host:port`. It's required that the SMTP server being supporting TLS.
 * `SMTP_VERITY_CERT`: Tell whether the SMTP certificate should be validated against top level authorities. If you're using a self-signed certificate on the SMTP server, this value must be set to `false`.
+* `SMTP_AUTHENTICATION_ENABLED`: Boolean (default: `true`) indicating whether SMTP authentication is required or not. If true, the variables `SMTP_CLIENT_USERNAME` and `SMTP_CLIENT_PASSWORD` are used the perform the authentication.
 * `SMTP_CLIENT_USERNAME`: Set the username to connect to the SMTP server.
 * `SMTP_CLIENT_PASSWORD`: Set the password to connect to the SMTP server.
 * `CONTACT_REPLY_EMAIL`: Set an email address for the reply email. It's not necessary a valid email address; for example if don't want the user to reply the email, you can set something like `noreply@example.com`.
diff --git a/main.go b/main.go
index 49bb8a5..f8a96e9 100644
--- a/main.go
+++ b/main.go
@@ -76,6 +76,7 @@ func main() {
 	viper.SetDefault("CONTACT_REPLY_BCC_EMAIL", "contact@company.com")
 	viper.SetDefault("EMAIL_SUBJECT", "Thanks to try our product")
 	viper.SetDefault("DEMO_URL", "http://company.com/product-demo")
+	viper.SetDefault("SMTP_AUTHENTICATION_ENABLED", true)
 
 	host := os.Getenv("HOST")
 	port := os.Getenv("PORT")
diff --git a/sendmail.go b/sendmail.go
index 51b3622..e58c098 100644
--- a/sendmail.go
+++ b/sendmail.go
@@ -97,7 +97,7 @@ func (m *SendMailRequest) Execute() error {
 	defer smtpClient.Quit()
 
 	// Authenticate if configured
-	if viper.GetString("SMTP_CLIENT_USERNAME") != "" {
+	if viper.GetBool("SMTP_AUTHENTICATION_ENABLED") {
 		smtpClientAuth := smtp.PlainAuth("",
 			viper.GetString("SMTP_CLIENT_USERNAME"),
 			viper.GetString("SMTP_CLIENT_PASSWORD"),