Every WordPress email through Notix, with no plugin of ours to install.

1. Choose the Other SMTP mailer.

   In WP Mail SMTP, that is Settings, General, Mailer. In FluentSMTP, add a connection and pick Other SMTP. Neither plugin lists Notix by name, and the generic option is all the relay needs.

2. Fill in the relay values.

   Encryption SSL goes with port 465 (TLS from the first byte); encryption TLS goes with port 587 (STARTTLS). Leave Auto TLS on and authentication on. The username is the fixed string notix; the password is the API key.

   WP Mail SMTPFluentSMTP

   WP Mail SMTP, Other SMTP mailer

   WP Mail SMTP, Settings, General, Mailer: Other SMTP
   
   SMTP Host:       smtp.usenotix.dev
   Encryption:      SSL           (or TLS, with the port below set to 587)
   SMTP Port:       465           (587 with TLS)
   Auto TLS:        on
   Authentication:  on
   SMTP Username:   notix
   SMTP Password:   an API key from the Notix dashboard, API keys
   
   From Email:      receipts@acme.com   (a domain you verified in Notix)
   Force From Email: on
   

3. Set and force the From address.

   Put an address on your verified domain in the From Email field and turn on the force option. Without it, WooCommerce, Contact Form 7 and WordPress itself each supply their own From, and any one of them on an unverified domain is refused.

4. Send the test email.

   Both plugins have a test tab. A successful send appears in the Notix dashboard within seconds with its status; a refused one shows the relay’s reason in the plugin’s log, and it is almost always the From domain.

<?php
/**
 * Plugin Name: Notix SMTP
 * Description: Sends every wp_mail() through the Notix SMTP relay.
 *
 * Save as wp-content/mu-plugins/notix-smtp.php. Must-use plugins load
 * before everything else and cannot be switched off from the admin.
 */

add_action( 'phpmailer_init', function ( $phpmailer ) {
    $phpmailer->isSMTP();
    $phpmailer->Host       = 'smtp.usenotix.dev';
    $phpmailer->Port       = 465;
    $phpmailer->SMTPSecure = 'ssl';          // 'tls' with Port 587
    $phpmailer->SMTPAuth   = true;
    $phpmailer->Username   = 'notix';
    // The password is a Notix API key. Read it from the environment or a
    // constant you define in wp-config.php; never commit it.
    $phpmailer->Password   = defined( 'NOTIX_API_KEY' ) ? NOTIX_API_KEY : getenv( 'NOTIX_API_KEY' );
} );

// WordPress defaults the sender to wordpress@your-site-host, which is not a
// domain you verified in Notix. Force an address that is.
add_filter( 'wp_mail_from', fn () => 'receipts@acme.com' );
add_filter( 'wp_mail_from_name', fn () => 'Acme' );

<?php
// Optional, in the same mu-plugin: log the relay's reason when a send is
// refused, instead of wp_mail() returning false in silence.
add_action( 'wp_mail_failed', function ( $error ) {
    error_log( 'notix smtp: ' . $error->get_error_message() );
} );

Define NOTIX_API_KEY in wp-config.php or export it in the PHP environment. The same PHPMailer values are the ones the PHP guide uses outside WordPress.