Mailgate SMTP

A WP must-use plugin that sends all mail through an SMTP server configured with constants in wp-config.php. It has no admin screen and stores nothing in the database, so it can not be changed or disabled from the admin panel.

What it does

• Sends every message that goes through wp_mail (WP core, Woo, form plugins) over SMTP with authentication.
• Applies its settings at the last phpmailer_init priority, so sender changes made by other plugins or by Woo are overridden right before sending.
• Uses the SMTP mailbox as the sender in the From header and in the envelope. SMTP servers usually reject senders that do not belong to the authenticated mailbox.
• Keeps the sender name set by a plugin, for example the Woo store name. The core default sender name is replaced by TORWALD45_MAILGATE_FROM_NAME or by the site name.
• Moves the original sender address to Reply-To when a plugin set a different sender and no Reply-To, so replies reach the intended address. The core default sender address is not used as Reply-To.
• Logs failed deliveries without credentials.

Requirements

• PHP 7.4 or newer
• WP 5.5 or newer (PHPMailer 6)
• An SMTP account

Installation

Copy the file to the must-use plugins directory:

wp-content/mu-plugins/torwald45-mailgate.php

Install a pinned release and verify its checksum. Example for v1.0, checksums of all releases are listed in CHANGELOG.md:

curl -fsSL -o torwald45-mailgate.php https://raw.githubusercontent.com/synchronicity-one/wp-torwald45-mailgate/v1.0/torwald45-mailgate.php
echo "8b830655db782f1aba18bac819e897dc266d3919f2e0f3d5072f36692eca7230  torwald45-mailgate.php" | sha256sum -c -

The file must be readable by the PHP process.

Configuration

Required constants in wp-config.php:

define( 'TORWALD45_MAILGATE_HOST', 'mail.example.com' );
define( 'TORWALD45_MAILGATE_USER', 'noreply@example.com' );
define( 'TORWALD45_MAILGATE_PASS', 'secret' );

Optional constants:

With WP-CLI:

wp config set TORWALD45_MAILGATE_HOST mail.example.com
wp config set TORWALD45_MAILGATE_PORT 587 --raw
wp config set TORWALD45_MAILGATE_USER noreply@example.com
wp config set TORWALD45_MAILGATE_PASS 'secret'

When a required constant is missing or a value is invalid, the plugin leaves mail unchanged and logs a warning.

WP-CLI commands

wp torwald45-mailgate status
wp torwald45-mailgate test recipient@example.com

status prints the configuration without the password. It warns when wp_mail is replaced by other code or when another mail plugin is active.

test sends a test message and prints the SMTP dialogue. Credentials in the dialogue are masked by PHPMailer.

Logging

Entries start with [torwald45-mailgate]. By default they go to the PHP error log, in Docker images usually the container log. Define TORWALD45_MAILGATE_LOG_FILE to write to a file instead. When the file is not writable, the entry goes to the PHP error log.

Compatibility

• Do not run it together with other SMTP plugins. Plugins that replace wp_mail bypass phpmailer_init and therefore this plugin.
• Mail sent without wp_mail is not covered.

License

GPL-2.0-or-later. See LICENSE.