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

In WHMCS 8.0 and later, this includes Mailgun, SendGrid, SparkPost, and SMTP with or without Google. WHMCS 8.6 added support for Microsoft services. For more information, see:

• Configuring WHMCS to use SMTP for WHMCS 8+
• Setting Up Google As Your Mail Service Provider
• Setting Up Microsoft As Your Mail Service Provider
• Mail Provider Integrations

To configure Mailgun, SendGrid, or SparkPost, go to the Mail tab at Configuration > System Settings > General Settings.

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.