Plunk

Plunk is the open-source Loops/Resend alternative — send transactional emails, manage contacts with custom data, trigger event-based email sequences, and run broadcast campaigns, all via a clean REST API. For agents: POST /track to log a contact event (triggers automations), POST /send to send a one-off transactional email, and POST /contacts to create/update contacts with custom metadata. Self-hosted on Docker for full data control. 4k+ GitHub stars for an open-source email platform. Confidence is docs-derived.

REST API at api.useplunk.com. Send email: POST /v1/send body: {"to": "user@example.com", "subject": "Welcome!", "body": "<h1>Hello</h1>", "from": "noreply@yourdomain.com"}. Response: {"success": true, "emails": [{"contact": {...}, "email": {...}}]}. Track event: POST /v1/track body: {"event": "user-signed-up", "email": "user@example.com", "data": {"plan": "pro"}}. Contact management: POST /v1/contacts/create body: {"email": "user@example.com", "subscribed": true, "data": {"name": "Alice"}}. GET /v1/contacts lists contacts. GET /v1/contacts/{id} retrieves contact. DELETE /v1/contacts/{id} deletes contact. Broadcasts: POST /v1/broadcasts/send sends a campaign to all subscribed contacts. Email templates: configure in Plunk dashboard; send with POST /v1/transactional/{template_id}.

API key auth: Authorization: Bearer {api_key}. Keys from Plunk dashboard → Settings → API Keys (secret key for server-side; public key for client-side event tracking). HTTPS enforced. No OAuth2. Domain verification: add DNS records (SPF, DKIM) for custom from domain. Self-hosted: manage your own secrets + SMTP/SES configuration. Rate limits: Plunk Cloud applies per plan; self-hosted limited only by SMTP provider.

Send reliability: Plunk uses AWS SES or configurable SMTP — delivery depends on sender reputation + domain verification. Bounce handling: automatic — Plunk processes SES bounce/complaint notifications. Open/click tracking: tracked via redirect links and tracking pixels (configurable). Event triggers: POST /v1/track fires automations configured in Plunk dashboard (sequences, tags). Async: email sending is asynchronous — POST /v1/send returns immediately on queue; final delivery not guaranteed at API response time. Self-hosted reliability: requires SMTP/SES configuration + PostgreSQL for data persistence.

docs.useplunk.com covers REST API reference, event tracking guide, contact management, broadcast campaigns, and self-hosting with Docker. Getting started: sign up, verify domain, POST /v1/send — first email in under 10 minutes. Plunk Cloud: free tier (3k emails/month). Self-hosted: Docker with SMTP config. GitHub: useplunk/plunk (4k+ stars). Community via Discord (small but active). Good documentation for an open-source project; REST API reference is clear and concise.