WHMCS comes with the ability to send emails out of the box, without any additional configuration, using PHP mail(). While this works in most cases, other mail providers may give you a better experience. 

In WHMCS 8.0 and later, this includes Mailgun, SendGrid, and SparkPost, in addition to SMTP.

For information about using SMTP as your Mail Provider, see Configuring WHMCS to use SMTP for WHMCS 8+ or Setting Up Google As Your Mail Service Provider.

To configure Mailgun, SendGrid, or SparkPost, navigate to Configuration > System Settings > General Settings > Mail.

Next to Mail Provider, click Configure Mail Provider.

Select your desired Mail Provider from the list.

In the new window, specify the settings for your provider. Remember that incorrect settings will prevent mail from sending in WHMCS.

Click Test Configuration at the bottom left of the window to make sure that you entered everything correctly. 

Once you see a success message, click Save Changes. WHMCS will now use your chosen Mail Provider to send mail.