Clusters
Find your cluster's connection details for SMTP and HTTP email sending.
Overview
A cluster is a group of one or more MTAs (mail transfer agents) that form your dedicated email sending infrastructure. Smaller deployments use a single MTA, while higher-volume deployments use multiple MTAs for increased throughput and redundancy.
Each cluster has its own SMTP and HTTP endpoints, domains, and credentials.
Finding Connection Details
To configure your application to send email, you need your cluster's connection details.
- Navigate to the Cluster page from the sidebar
- View the Connection Details card on the right
The connection details include:
| Field | Example | Description |
|---|---|---|
| HTTP | https://c-your-cluster-id.unmta.net/api/inject/v1 | HTTPS endpoint for HTTP message injection |
| SMTP Host | c-your-cluster-id.unmta.net | Hostname for SMTP connections |
Your cluster's hostname will be unique. Replace c-your-cluster-id.unmta.net
in the examples with the value from your dashboard.
SMTP Ports
unMTA accepts SMTP connections on three ports. All ports require STARTTLS encryption.
| Port | Use Case |
|---|---|
| 587 | Recommended for most applications. Standard submission port. |
| 25 | Standard SMTP port. May be blocked by some cloud providers. |
| 2587 | Alternative port if 587 is blocked by your network or firewall. |
Port 587 is the recommended choice for application email. Some cloud providers (AWS, GCP, Azure) block outbound port 25 by default to prevent spam.
Connecting Your Application
Use the connection details with your credentials to send email.
SMTP Example
swaks --to recipient@example.com \
--from sender@yourdomain.com \
--server c-your-cluster-id.unmta.net:587 \
--tls \
--auth-user your-credential \
--auth-password your-password \
--header "Subject: Test" \
--body "Hello from unMTA"HTTP Example
curl -X POST https://c-your-cluster-id.unmta.net/api/inject/v1 \
-u "username:password" \
-H "Content-Type: application/json" \
-d '{
"envelope_sender": "bounces@yourdomain.com",
"recipients": ["user@example.com"],
"content": {
"from": "sender@yourdomain.com",
"subject": "Hello",
"text_body": "Hello from unMTA"
}
}'For complete message options, see Sending Messages.
Rate Limits
Some clusters may have a message rate limit configured. This controls how many messages each MTA in the cluster will send within a given time window.
For most clusters, the rate limit is set to Unlimited. Rate limits are typically only applied during customer trials or IP warming scenarios where controlled sending volumes are important for building sender reputation.
When a rate limit is active, it is displayed on the Cluster page in the dashboard. The rate is shown per MTA (e.g., 500/hr per MTA).
Rate limits are managed by administrators and are read-only for customers. Messages are never rejected when a rate limit is exceeded—they are held in a scheduled queue and delivered when the rate window allows.
Multi-MTA Clusters
Clusters with multiple MTAs provide increased throughput and fault tolerance. Your application connects to the same hostname regardless of how many MTAs are in the cluster - the infrastructure handles distribution automatically.
DNS load balancing - Traffic is distributed across MTAs using DNS. Each connection resolves to one of the available MTAs in your cluster.
Automatic health monitoring - MTAs are continuously monitored. If an MTA becomes unavailable, it's automatically removed from the DNS rotation. When the MTA recovers, it's restored to the rotation.
No application changes - Scaling from one MTA to multiple (or vice versa) requires no changes to your application configuration. The cluster hostname remains the same.
API Reference
To list your clusters programmatically, see the Clusters API documentation.