Send WHMCS emails through Resend

Connect WHMCS to the Resend API to send invoices, password resets, service notifications and support emails while keeping your existing WHMCS templates and workflow.

Developed by Duplika, this free, open-source mail provider preserves the email content, template language, sender and recipients supplied by WHMCS. Its configuration screen, help text and error messages are in English.

Features

• Send transactional emails through the Resend HTTPS API.
• Preserve HTML and plain-text content, To, CC, BCC and Reply-To.
• Send invoice PDFs and other supported local or in-memory attachments.
• Preserve supported conversation headers for support-ticket replies.
• Retry temporary failures using the same idempotency key to help prevent duplicate sends during retries.
• Configure a per-attempt timeout and up to three total attempts, within a shared 25-second time budget.
• Test the configuration by sending an email to the currently logged-in WHMCS administrator.
• Record the Resend email ID in the WHMCS Activity Log without including API keys or email contents in the module's log entries.
• Install without Composer, core-file changes, hooks or database migrations.

Requirements

• Tested with WHMCS 9.0.9 and PHP 8.3. Other version combinations have not been verified.
• PHP cURL enabled for both web requests and cron, current CA certificates and outbound HTTPS access to api.resend.com on port 443.
• A Resend account with all sender domains verified, including domains used by support departments.
• A dedicated Resend API key with Sending access permission for those domains.
• Sufficient transactional sending capacity in your Resend plan.

The module is free under the MIT license. Resend service usage is subject to your Resend plan and limits.

Installation

1. Download and extract the installation ZIP. Save your current mail-provider settings before making changes.
2. Upload only the modules folder to your WHMCS root directory. The main file must be located at modules/mail/Resend/Resend.php. Preserve the capitalization of Resend.
3. Open Configuration → System Settings → General Settings → Mail → Configure Mail Provider.
4. Select Resend API and enter your Resend API key.
5. Set Test sender to an email address on a domain verified in Resend. Start with Timeout per attempt: 10 and Maximum attempts: 2.
6. Click Test Configuration. This sends a real email to the administrator currently logged in to WHMCS. Confirm receipt and check the result in Resend, then save the configuration.
7. Test an invoice with a PDF, a notification and a support reply using accounts you control before enabling the provider for production use.

This is a mail provider, not a module that needs activation under Addon Modules. The Test sender setting is used only for the configuration test; normal emails retain the sender and recipients supplied by WHMCS.

Updating and troubleshooting

To update, back up and replace modules/mail/Resend/, then reopen Configure Mail Provider. Your saved API key and settings are retained.

Look for Resend API in the WHMCS Activity Log and use the recorded email ID to inspect the message in Resend. API acceptance does not guarantee final delivery; check the delivery result in Resend before manually resending an uncertain message.

This version handles outgoing email only. It does not synchronize marketing contacts, receive delivery webhooks or change incoming email and support piping. It does not provide a persistent retry queue or automatic SMTP fallback.

Independent integration developed by Duplika. Not an official WHMCS or Resend product.