mortensen/smtp-resend-monitor

WordPress mu-plugin that monitors SMTP failures and sends alerts via Resend HTTP API.

v1.0.5 2026-04-17 12:18 UTC

Requires

• php: >=8.0
• composer/installers: ^2.0

MIT

• Mortensen

README

WordPress mu-plugin that monitors SMTP failures and sends alerts via Resend HTTP API.

The Problem

When SMTP fails on a WordPress site, there is no way to be notified — because the notification mechanism (email) is itself broken. Sites can go months with broken SMTP until someone reports that forms aren't working.

How It Works

1. Real-time detection: Hooks into wp_mail_failed to catch SMTP failures as they happen
2. Proactive health-check: Sends a test email every 24 hours via WP-Cron — if it fails, you get an alert
3. Alert via HTTP: Bypasses the broken SMTP entirely by sending alerts through Resend's HTTP API
4. Cooldown: Maximum 1 alert per 6 hours per site to prevent spam

Works with any SMTP setup: WP Mail SMTP plugin, roots/acorn-mail, or native WordPress.

Requirements

• PHP >= 8.0
• WordPress
• A Resend account with a verified domain

Installation

composer require mortensen/smtp-resend-monitor

The package type is wordpress-muplugin, so Composer places it automatically in mu-plugins/.

Configuration

Add these variables to your .env file (Bedrock) or define them in wp-config.php:

Variable	Required	Description
SMTP_MONITOR_RESEND_API_KEY	Yes	Your Resend API key
SMTP_MONITOR_ALERT_TO	Yes	Alert recipient email
SMTP_MONITOR_ALERT_FROM	Yes	Alert sender email (must be from a verified Resend domain)

Bedrock (.env)

SMTP_MONITOR_RESEND_API_KEY=re_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
SMTP_MONITOR_ALERT_TO=[e-mail]
SMTP_MONITOR_ALERT_FROM=[e-mail]

wp-config.php

define('SMTP_MONITOR_RESEND_API_KEY', 're_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx');
define('SMTP_MONITOR_ALERT_TO', '[e-mail]');
define('SMTP_MONITOR_ALERT_FROM', '[e-mail]');

How It Detects Failures

Real-time (wp_mail_failed)

Every time wp_mail() fails, WordPress fires the wp_mail_failed action with a WP_Error object. This plugin hooks into it and sends an alert via Resend's HTTP API.

Health-check (WP-Cron)

Every 24 hours, the plugin sends a test email via wp_mail() to the site's admin email. If the send fails, it triggers an alert. This catches silent failures where no user activity triggers wp_mail().

Alert Email

You receive an email with:

• Site name and URL
• Error message from WordPress
• Detection type: "Real-time" or "Health-check"
• Timestamp

Cooldown

To prevent alert spam (e.g., a form being submitted repeatedly while SMTP is broken), only 1 alert is sent per 6 hours per site.

Behavior When Not Configured

If SMTP_MONITOR_RESEND_API_KEY is not set, the plugin does nothing — no errors, no warnings, no performance impact.

License

MIT