Lettr - Email API

Send transactional and marketing emails from your WordPress site using the Lettr email API. Lettr replaces the default WordPress email system with a reliable, developer-friendly API that delivers emails to inboxes — not spam folders. Why Lettr?

• Reliable email delivery — Built on battle-tested infrastructure with SPF, DKIM, and DMARC authentication out of the box.
• Simple setup — Connect your WordPress site with a single API key. No SMTP configuration needed.
• Transactional email at scale — Send password resets, order confirmations, notifications, and more through the Lettr email API.
• Real-time tracking — Monitor opens, clicks, bounces, and deliverability from the Lettr dashboard.
• Templates & personalization — Use the Lettr drag-and-drop editor and merge tags for dynamic content.
• Developer-first — RESTful API, detailed documentation, and SDKs for PHP, Node.js, Python, Go, Rust, Java, and Laravel.

How it works This plugin overrides the default WordPress wp_mail() function and routes all outgoing emails through the Lettr API. This means every email your site sends — including emails from WooCommerce, contact form plugins, and any other plugin — is delivered through Lettr automatically. Links

• Lettr Website
• Documentation
• API Reference

1.4.0

• Add list_folders() — the folders templates are filed into, with each folder's purpose and template count. Nothing else in the API returns a folder id, so without it a caller either omits folder_id and accepts whichever folder the API picks, or hardcodes an integer read out of an app URL.
• create_template() accepts purpose (transactional or campaign), and list_templates() accepts purpose and folder_id filters. A campaign can only send a template whose purpose is campaign, and the purpose cannot be changed after creation.
• Add the lettr_idempotency_key filter. Return a stable string and a retried wp_mail() replays the original send instead of delivering a second email. Opt-in by design: deriving a key from the payload would silently collapse two legitimately identical notifications into one. Lettr_Api::send_email() also takes the key as a second argument.
• Template responses now carry preparation_status (pending, ready, failed) — documented; no client change was needed, since responses pass through decoded.

1.3.0

• Add bulk topic subscription API client methods (bulk_subscribe_audience_contacts_to_topics, bulk_unsubscribe_audience_contacts_from_topics).
• bulk_create_audience_contacts() now documents the per-contact contacts request shape, the batch-wide list_ids / topics / update_existing options, and the updated / error_count / errors / contacts response fields. No call signature changed — existing payloads send exactly as before.
• Behaviour change from the Lettr API: creating a contact whose email already exists now returns HTTP 409, so create_audience_contact() fails with a WP_Error code of lettr_api_resource_already_exists (status 409). It previously returned HTTP 500, i.e. lettr_api_send_error (status 500). Custom code branching on the old code no longer matches, and a 409 here must not be retried.

1.2.0

• Add campaigns API client methods (list, show, events, send, schedule, unschedule).

1.1.0

• Add audience API client methods (lists, contacts, topics, properties, segments).
• Bump tested-up-to to WordPress 7.0.

1.0.0

• Initial release.