SendMailOS
The email platform for developers — send transactional and marketing email with high deliverability, one clean API.
SendMailOS is a developer-first email platform. One API sends transactional and marketing email, manages subscribers, verifies sending domains, and drives automated workflows — with delivery, opens, clicks, and bounces streamed back to you in real time.
Base URL & authentication
Every request goes to the versioned base URL and carries a Bearer token — either
an API key (sk_live_… / sk_test_…) or an OAuth access token (oat_…,
scoped to a user's grant). Both are accepted on every /v1 endpoint.
curl https://api.sendmailos.com/v1/emails \
-H "Authorization: Bearer sk_live_your_key" \
-H "Content-Type: application/json" \
-H "Idempotency-Key: $(uuidgen)" \
-d '{
"from": "You <[email protected]>",
"to": "[email protected]",
"subject": "Welcome to SendMailOS",
"html": "<h1>You are in.</h1>"
}'A successful send returns the email's id and its lifecycle status. Reads,
events, and cancellation live under the same /v1/emails resource.
Everything at a glance
Send an email
POST /v1/emails — a single message, a batch, or one template to many recipients.
Verify a domain
BYODKIM signing + custom MAIL FROM, with the exact DNS records to publish.
Subscribers
Create and manage contacts, tags, and topic subscriptions.
Webhooks
Receive svix-compatible, signed delivery / open / click / bounce events.
Broadcasts
One-to-many marketing sends across an audience.
Workflows
Automate multi-step journeys triggered by events.
Built for reliability
- Idempotency — pass an
Idempotency-Keyon any write; a replay with the same key returns the cached response, so retries never double-send. - One error envelope — every failure is
{ request_id, error: { code, message, retryable } }. - High deliverability — BYODKIM, custom Return-Path, and warmed sending pools keep you out of spam.
Pick a resource from the sidebar to see its full reference — parameters, request and response schemas, and copy-paste code samples in curl, Node, and Python.