Email Deliverability
How your emails get delivered
Synapse handles email delivery through Resend, with automatic authentication setup, bounce handling, and full lifecycle tracking. Here is exactly how it works.
Delivery via Resend
Synapse sends email through the Resend API. Synapse is the intelligence layer — it decides what to send, when, and to whom based on your flows and templates. Resend handles the actual SMTP delivery.
This architecture means Synapse never runs its own SMTP servers. Resend handles IP reputation, delivery optimization, and ISP relationships.
Sender domain authentication
When you add a custom sender domain in Synapse, the platform generates the DNS records needed for email authentication. You add them to your DNS provider, and Synapse verifies they are live before marking the domain as active.
Sender Policy Framework. Authorizes Resend's servers to send email on behalf of your domain. Synapse generates the correct include directive.
DomainKeys Identified Mail. Cryptographic signature on every email proving it was not tampered with in transit. Synapse sets up the CNAME record pointing to Resend's signing key.
Domain-based Message Authentication, Reporting, and Conformance. Tells receiving servers what to do with messages that fail SPF or DKIM checks. Synapse generates a TXT record with your policy.
For Cloudflare-managed domains, Synapse supports one-click DNS setup via Domain Connect. For other providers, the dashboard auto-detects your DNS provider and links directly to the management panel with setup instructions.
Automatic bounce and complaint handling
Sender reputation is the most important factor in email deliverability. Synapse protects it automatically.
On bounce
When an email bounces, Synapse automatically sets the contact's subscription status to unsubscribed. No more emails will be sent to that address until the status is manually changed. The bounce event is recorded in the contact's timeline and the email log.
On complaint
When a recipient marks an email as spam, Synapse immediately unsubscribes the contact using the same mechanism. Continuing to send to contacts who have complained damages sender reputation across the entire sending domain.
Webhook verification
All delivery event webhooks from Resend are verified using Svix signature verification before processing. In production, events with invalid signatures are rejected. This prevents spoofed delivery events from corrupting contact subscription state.
Full delivery lifecycle tracking
Every email generates a log record. Synapse tracks 9 delivery event types from the moment an email is dispatched until the recipient interacts with it.
| Event | What it means |
|---|---|
| sent | Email accepted by the delivery provider |
| delivered | Email reached the recipient's inbox |
| delivery_delayed | Delivery temporarily deferred by the receiving server |
| opened | Recipient opened the email |
| clicked | Recipient clicked a link in the email |
| bounced | Email bounced. Contact is automatically unsubscribed to protect sender reputation. |
| complained | Recipient marked the email as spam. Contact is automatically unsubscribed. |
| failed | Delivery failed permanently |
| suppressed | Email suppressed to protect deliverability |
Open and click events include parsed engagement metadata: email client name (Gmail, Outlook, Apple Mail, etc.), device type, and operating system. This data is stored in the email log for analytics.
Shadow mode
When shadow mode is enabled, Synapse processes the entire email pipeline — template rendering, flow logic, contact resolution — but does not actually send the email. The send is logged as a shadow delivery.
Use shadow mode to validate your flows and templates in a production-like environment without sending real emails. The test/live environment isolation also provides this: test mode sends real emails, but only to verified team addresses.
Circuit breaker protection
Email delivery calls to Resend are wrapped in a circuit breaker. If Resend experiences an outage, Synapse detects the failure pattern and stops making calls to the provider until it recovers.
- 5-failure threshold before the circuit opens
- 60-second recovery timeout before retrying
- Redis-backed state shared across all API instances
What we do not have yet
We believe in being upfront about the current state of the platform. These capabilities are on our roadmap but are not available today.
| Capability | Current state |
|---|---|
| Dedicated IP for sending | Emails are sent through Resend's shared IP pool. Dedicated IPs are planned. |
| IP warming | A planning document exists but the feature is not implemented. |
Start sending with confidence
Free tier includes 2,000 emails per month with full delivery tracking. No credit card required.