WordPress Subscriber Sync

What this is for#

The simplest WordPress ↔ AcelleMail integration: when someone registers on your WordPress site, add them to an AcelleMail list. Twenty lines of PHP, no plugin dependency, real-time.

This article is Pattern 1 from the WordPress + WooCommerce integration overview. If you're connecting WooCommerce purchases (not just user registrations), see WooCommerce Post-Purchase Emails instead.

Prerequisites#

• A working AcelleMail install
• An AcelleMail API token
• The UID of the list you want to add subscribers to (in AcelleMail: Lists → Overview → click the list → URL contains the UID)
• WordPress 5.5+ (we'll use wp_remote_post + wp_json_encode)

Step 1 — Store credentials in wp-config.php#

Never put API tokens in your theme. Put them in wp-config.php so they're outside the version-controlled theme code:

// /path/to/wp-config.php
define('ACELLEMAIL_BASE_URL',  'https://mail.example.com');
define('ACELLEMAIL_API_TOKEN', 'your-token-here');
define('ACELLEMAIL_LIST_UID',  'your-list-uid-here');

Step 2 — Create a must-use plugin#

Don't paste into functions.php — it'll vanish on theme switch. Create a must-use plugin instead (wp-content/mu-plugins/ auto-loads every plugin in that directory regardless of theme):

# WordPress doesn't create mu-plugins/ by default; mkdir it if needed
sudo mkdir -p /path/to/wp-content/mu-plugins
sudo nano /path/to/wp-content/mu-plugins/acellemail-subscriber-sync.php

<?php
/**
 * Plugin Name: AcelleMail Subscriber Sync
 * Description: Push new WP user_register events to AcelleMail
 * Version:     1.0.0
 */

if (!defined('ACELLEMAIL_API_TOKEN') ||
    !defined('ACELLEMAIL_LIST_UID') ||
    !defined('ACELLEMAIL_BASE_URL')) {
    return; // Constants not set in wp-config.php; skip silently
}

add_action('user_register', function ($user_id) {
    $user = get_user_by('id', $user_id);
    if (!$user || !is_email($user->user_email)) return;

    $response = wp_remote_post(ACELLEMAIL_BASE_URL . '/api/v1/subscribers', [
        'headers' => [
            'Authorization' => 'Bearer ' . ACELLEMAIL_API_TOKEN,
            'Accept'        => 'application/json',
            'Content-Type'  => 'application/json',
        ],
        'body' => wp_json_encode([
            'MAIL_LIST_UID' => ACELLEMAIL_LIST_UID,
            'EMAIL'         => $user->user_email,
            'FIRST_NAME'    => $user->first_name ?: '',
            'LAST_NAME'     => $user->last_name ?: '',
        ]),
        'timeout'  => 10,
        'blocking' => false,   // fire and forget — don't slow down user signup
    ]);

    // With blocking => false, $response is always WP_Error-ish + we never see status.
    // Use blocking => true + WP_DEBUG_LOG for first-time setup, then flip back.
});

Step 3 — Test it#

Register a test user via WordPress Admin → Users → Add New. Check AcelleMail:

• Lists → your list → Subscribers should show the test user within 5 seconds
• If WP_DEBUG_LOG is on, wp-content/debug.log should not show any PHP errors
• AcelleMail's storage/logs/laravel.log should not show any 422/500 errors

If the subscriber doesn't appear:

// Temporarily flip 'blocking' => true and 'timeout' => 30, then log the response
$body = wp_remote_retrieve_body($response);
error_log("[AcelleMail sync] HTTP " . wp_remote_retrieve_response_code($response) . " — $body");

Common debug results:

• 401 Unauthenticated → wrong API token in wp-config.php
• 422 The MAIL_LIST_UID is invalid → wrong list UID
• cURL error 60: SSL → self-signed cert on AcelleMail; either fix the cert or temporarily 'sslverify' => false

Step 4 — Field mapping#

Acelle subscriber fields are stored per-list. The default fields every list has:

If your AcelleMail list has additional custom fields, add them to the body:

'COMPANY'        => get_user_meta($user_id, 'company_name', true),
'SIGNUP_SOURCE'  => 'wordpress',
'WP_ROLE'        => implode(',', $user->roles),

Custom field keys are UPPERCASE matching what you defined in AcelleMail (Lists → your list → Fields).

Step 5 — Differentiate by role / signup source#

A common refinement: subscribers from different sources go to different lists.

add_action('user_register', function ($user_id) {
    $user = get_user_by('id', $user_id);
    if (!$user) return;

    // Customers go to "Newsletter — Customers"; everyone else to "Newsletter — General"
    $list_uid = in_array('customer', $user->roles, true)
        ? CUSTOMER_LIST_UID
        : GENERAL_LIST_UID;

    wp_remote_post(/* ... use $list_uid instead of constant ... */);
});

Optional — opt-in checkbox at registration#

Don't auto-subscribe everyone — many compliance regimes (GDPR, CASL, etc.) require explicit consent. Add a checkbox to the WP registration form:

add_action('register_form', function () {
    ?>
    <p>
        <label>
            <input type="checkbox" name="newsletter_optin" value="1" />
            Subscribe to our newsletter
        </label>
    </p>
    <?php
});

add_action('user_register', function ($user_id) {
    if (empty($_POST['newsletter_optin'])) {
        // User didn't opt in — skip
        return;
    }
    // ... your wp_remote_post call ...
});

Combine with double opt-in at the AcelleMail list level for maximum compliance — Acelle then sends its own confirmation email after the WP signup.

Optional — handle subscription updates#

When a user updates their email in WordPress, push the change to AcelleMail:

add_action('profile_update', function ($user_id, $old_user_data) {
    $user = get_user_by('id', $user_id);
    if ($user->user_email === $old_user_data->user_email) return; // no email change

    // Update AcelleMail subscriber's email
    // First, find by old email:
    $find = wp_remote_get(
        ACELLEMAIL_BASE_URL . '/api/v1/subscribers/email/' . urlencode($old_user_data->user_email),
        ['headers' => ['Authorization' => 'Bearer ' . ACELLEMAIL_API_TOKEN, 'Accept' => 'application/json']]
    );

    $sub = json_decode(wp_remote_retrieve_body($find), true);
    if (!$sub || empty($sub['id'])) return;

    wp_remote_request(
        ACELLEMAIL_BASE_URL . '/api/v1/subscribers/' . $sub['id'],
        [
            'method' => 'PATCH',
            'headers' => ['Authorization' => 'Bearer ' . ACELLEMAIL_API_TOKEN, 'Accept' => 'application/json', 'Content-Type' => 'application/json'],
            'body' => wp_json_encode(['EMAIL' => $user->user_email]),
        ]
    );
}, 10, 2);

Common issues#

FAQ#

Will this work with BuddyPress / bbPress / membership plugins? Yes — their account-creation hooks fire user_register automatically. Test against your specific plugin to confirm.

Can I delete subscribers from AcelleMail when a WP user is deleted? Yes — hook delete_user and POST to DELETE /api/v1/subscribers/{id} (or use the /by-email lookup variant). For GDPR right-to-erasure compliance, this is recommended.

What if a user already exists in AcelleMail? The API treats the request as upsert — the existing subscriber is updated with the new field values. No duplicate created.

Performance at scale? Each wp_remote_post('blocking' => false) returns in < 1ms. Even 10,000 registrations per hour is negligible WP overhead. The AcelleMail side handles the writes via its queue.

HTTPS-only? Yes, always. Plain HTTP for the API call exposes your token. AcelleMail should be on HTTPS anyway per hardening checklist.

Related articles#

• Integrating AcelleMail with WordPress and WooCommerce — the master overview (this is Pattern 1)
• WooCommerce Post-Purchase Emails — for purchase events instead of user registration
• Getting Started with the AcelleMail REST API — token + API fundamentals
• REST API Authentication and Endpoints — full subscriber API reference
• Double Opt-In vs Single Opt-In — compliance decision
• Importing Contacts — for bulk-importing existing WP users
• Zapier Integration Guide — no-code alternative
• GDPR Compliance Guide for Email Marketing — consent requirements