Cold email infrastructure that just works.
Mailbox rotation, health monitoring, reply detection, and deliverability management — all through one API. The Resend for cold email.
curl -X POST https://zend.sh/api/v1/campaigns \
-H "Authorization: Bearer zk_live_..." \
-H "Content-Type: application/json" \
-d '{"name": "Q1 Outreach", "stop_on_reply": true}'Connect your inboxes in 2 minutes.
Google Workspace, Outlook, or any SMTP provider.
Built for cold email at scale
Everything you need to send, monitor, and optimize cold email campaigns through one API.
Mailbox Rotation
Distribute sends across healthy accounts with round-robin, health-weighted, or least-used rotation strategies. No single mailbox takes all the load.
Strategy: health-weighted
outreach1@co.com → 40% (health: 95)
outreach2@co.com → 35% (health: 87)
outreach3@co.com → 25% (health: 72)Health Monitoring
Auto-pause mailboxes when bounce rate exceeds 2% or complaint rate exceeds 0.3%. Real-time health scores per account with automatic recovery.
● outreach1@co.com 95/100 active
● outreach2@co.com 87/100 active
▲ outreach3@co.com 72/100 warning
✗ outreach4@co.com 31/100 paused
→ bounce rate 3.2% (threshold: 2%)Reply Detection
IMAP polling with AI intent classification. Detect interested replies, meeting requests, out-of-office, and unsubscribes automatically.
New reply detected:
From: jane@acme.com
Intent: interested (0.94)
→ Campaign paused for lead
→ Webhook fired: email.repliedVariable Substitution
Personalize every email with lead fields and Handlebars templates. Fallback values ensure no blank fields slip through.
Hi {{first_name|there}},
I noticed {{company}} is hiring for
{{role|a growth role}}. We help teams
like yours scale outbound 3x...The complete sending toolkit
Scheduling, webhooks, experimentation, and SDKs. Everything you need to build and optimize your outbound pipeline.
Send Pipeline
Timezone-aware scheduling, randomized delays between sends, and automatic CAN-SPAM unsubscribe headers on every email.
Webhooks
HMAC-signed event delivery for sent, bounced, replied, and complained events. Exponential backoff retry on failure.
A/B Testing
Weight-distributed variants per sequence step. Test subject lines, body copy, and send times to optimize reply rates.
SDKs
TypeScript and Python SDKs with full type definitions. Build your outbound automation in the language you already use.
REST API
Every action available through a clean REST API. Accounts, campaigns, leads, analytics, webhooks — all programmatically.
MCP Server
AI agents can manage campaigns via the Model Context Protocol. Claude, GPT, or any MCP-compatible agent works out of the box.
No Tracking Pixels
No open tracking or click wrapping by default. Tracking pixels hurt deliverability — we optimize for inbox placement.
Frequently Asked Questions
Everything you need to know about cold email infrastructure with zend.sh.