Intelligent Email Delivery
Most email infrastructure tells you it sent. SendLib tells you what happened, and gives you the controls to change it.
Warmup Plan
Pacing Rule
gmail.com → max 250/min
ramp 10%/day
API
priority: high
provider: Gmail
provider deferral
next: +30s
250 OK
Why email breaks at scale
Everything looks fine until volume makes it real.
Dev sends work. Problems surface when providers push back, reputation drifts, and one stream starts infecting another.
Traffic spike → throttle → cascading delay
A campaign launches, volume jumps, Gmail pushes back, and suddenly your password resets are delayed too.
Provider-aware pacing absorbs spikes. Transactional streams bypass campaign queues.
Campaign volume poisons transactional delivery
Marketing sends drag reputation down and your critical emails start landing in spam.
Stream isolation keeps transactional and campaign reputation, pacing, and queues fully separate.
“Email not received”, with no way to answer why
Support escalation. Engineering investigation. Hours spent. No signal anywhere in your stack.
Per-message event timelines show the exact provider response, failure reason, and every retry attempt.
Authentication drift breaks deliverability silently
A DNS change, a DKIM misalignment, a partial rollout. Performance degrades before anyone notices.
Domain verification workflows and policy guardrails catch auth issues early.
These are infrastructure problems. The fix starts with knowing exactly what happened.
Complete signal from every send
Every state. Every provider. Every reason.
From injection through every retry and deferral to final outcome, the full event trail lands on your webhook, not in a log file you’ll never read.
Send
POST /v1/transmissions
curl -X POST https://api.sendlib.com/v1/transmissions \
-H "Authorization: Bearer sk_live_..." \
-H "Idempotency-Key: inv-2026-1042" \
-H "Content-Type: application/json" \
-d '{
"from": "billing@yourapp.com",
"to": ["user@example.com"],
"subject": "Invoice #1042",
"html": "<p>Your invoice is attached.</p>",
"stream": "transactional"
}'Delivery event
Pushed to your webhook in real time
{
"type": "delivery",
"timestamp": "2026-02-24T14:22:08.445Z",
"message_id": "msg_01HP3GZK7E3D2...",
"stream": "transactional",
"recipient": "user@example.com",
"provider": "gmail",
"outcome": {
"status": "delivered",
"smtp_code": 250,
"reason": null
}
}Platform + expertise
Send it. Watch it. Shape it. Fix it.
Four capabilities covering the full lifecycle of email delivery. Use the platform solo, or pair it with consulting when reputation is on the line.
Send
One pipeline for REST and SMTP. Your transactional and campaign traffic stay isolated from day one.
- REST API and SMTP relay
- Idempotency-Key support on every request
- Template-ready payloads
- Separate transactional and campaign streams
Observe
See where each message went, why it deferred, and what the provider said.
- Real-time delivery and engagement events
- Typed webhooks with HMAC signing
- Per-message event timelines
- Bounce classification with provider context
Control
Pacing rules, IP warmup schedules, and suppression enforcement run automatically. You set the policy; the system holds it.
- Pacing rules per provider
- IP warmup workflows with progress tracking
- Automatic suppression enforcement
- Domain authentication and verification
Consult
Deliverability specialists who audit your setup, fix reputation problems, and stay accountable to results.
- Deliverability audit with a written remediation plan
- Sender reputation diagnosis and recovery
- DNS, SPF, DKIM, and DMARC review
- Ongoing inbox placement guidance
Built to hold under load
No shortcuts in the architecture.
Each of those four capabilities is built on infrastructure designed for production from day one, not patched in after the fact.
Async message pipeline
Every message is accepted and enqueued immediately. Workers handle delivery, retries, and lane routing without blocking your application.
Per-tenant isolation
One sender's volume, reputation, or failure never touches another. Rate limits, queue lanes, and sending pools are fully separated.
Provider-aware delivery
Gmail, Outlook, and Yahoo each behave differently. Pacing, retry timing, and warmup schedules adapt automatically per provider.
Idempotent by default
Safe to retry on any network failure without duplicating sends. Every request accepts an Idempotency-Key.
HMAC-signed tracking
Engagement attribution stays clean. Tracking tokens are cryptographically signed and bot-generated clicks are filtered out.
Message event flow
Full observability for every message
[
{ "type": "injection", "ts": "14:22:08.012" },
{ "type": "queued", "ts": "14:22:08.015",
"lane": "transactional", "priority": "high" },
{ "type": "attempt", "ts": "14:22:08.089",
"provider": "gmail", "mx": "alt1.gmail-smtp" },
{ "type": "tempfail", "ts": "14:22:09.201",
"reason": "421 try again later" },
{ "type": "attempt", "ts": "14:22:38.003",
"provider": "gmail", "mx": "alt2.gmail-smtp" },
{ "type": "delivery", "ts": "14:22:38.445",
"status": "250 OK" }
]Pacing policy
Provider-aware rate shaping
{
"type": "warmup",
"name": "new-domain-gmail-ramp",
"provider": "gmail.com",
"schedule": [
{ "day": 1, "max_per_hour": 50 },
{ "day": 7, "max_per_hour": 500 },
{ "day": 14, "max_per_hour": 2000 },
{ "day": 21, "max_per_hour": null }
],
"on_deferral": "slow_10_pct"
}Get started
Three steps to your first delivery event.
Credentials, domain, send. Events land on your webhook from the very first request.
Get your API key
Sign up and generate credentials from the dashboard.
Verify your domain
Add DNS records for SPF and DKIM. Auto-verified.
Send your first email
One request to the transmissions endpoint. Events arrive instantly.
Send your first email
One request. Full pipeline.
curl -X POST https://api.sendlib.com/v1/transmissions \
-H "Authorization: Bearer $SENDLIB_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"from": "hello@yourapp.com",
"to": ["user@example.com"],
"subject": "Welcome aboard",
"html": "<p>You are all set.</p>",
"stream": "transactional"
}'Start with the API. Add expertise when you need it.
Build on the platform yourself, or bring in the deliverability team when performance needs a specialist.