> ## Documentation Index
> Fetch the complete documentation index at: https://ahasend.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# SMTP Server Information

> Complete guide to AhaSend SMTP servers, connection settings, limits, and response codes

AhaSend provides high-performance SMTP relay servers for sending transactional emails. Our SMTP infrastructure supports standard protocols and offers reliable email delivery with comprehensive error handling.

<Info>
  **New to SMTP?** Check out our [SMTP credentials guide](/smtp/credentials) to get started, or try our [HTTP API](/send-api/send-email) for a more modern approach.
</Info>

## Server Endpoints

AhaSend operates SMTP servers in multiple regions for optimal performance:

<CardGroup cols={2}>
  <Card title="European Server" icon="earth-europe">
    **Hostname:** `send.ahasend.com`<br />
    **Location:** Germany (Primary)<br />
    **Best for:** European and global traffic
  </Card>

  <Card title="US Server" icon="earth-americas">
    **Hostname:** `send-us.ahasend.com`<br />
    **Location:** Oregon, USA<br />
    **Best for:** North American traffic
  </Card>
</CardGroup>

<Tip>
  **Server selection:** Choose the server closest to your application for best performance. Both servers offer identical functionality and delivery capabilities.
</Tip>

## Connection Settings

### Supported Ports & Encryption

All AhaSend SMTP servers support the following configurations:

```
Servers: send.ahasend.com | send-us.ahasend.com
Ports: 25, 587, 2525
Encryption: STARTTLS (required)
Authentication: PLAIN (required)
```

<Warning>
  **SMTPS not supported:** AhaSend does not support implicit SSL/TLS (SMTPS) on port 465. Use STARTTLS on ports 25, 587, or 2525 instead.
</Warning>

### Authentication Requirements

* **Authentication Method:** PLAIN authentication over STARTTLS
* **Username:** Your SMTP username from the dashboard
* **Password:** Your SMTP password from the dashboard
* **STARTTLS:** Required for all connections

<Note>
  **Get SMTP credentials:** Create SMTP credentials in your [AhaSend Dashboard](https://dash.ahasend.com) in the "Credentials" page or via the [API](/api-reference/smtp-credentials/create-smtp-credential).
</Note>

## Connection Limits & Specifications

Understanding AhaSend's SMTP limits helps you optimize your email sending:

<AccordionGroup>
  <Accordion title="Recipients Per Message" icon="users">
    **Limit:** 50 recipients maximum per message

    This is the maximum number of consecutive `RCPT TO` commands that can be issued for a single SMTP transaction.

    ```
    MAIL FROM: <sender@yourdomain.com>
    RCPT TO: <user1@example.com>
    RCPT TO: <user2@example.com>
    ... (up to 50 recipients)
    DATA
    ```

    <Tip>
      For bulk sending to more recipients, send multiple separate messages or use our [HTTP API](/send-api/send-email) with batch operations.
    </Tip>
  </Accordion>

  <Accordion title="Message Size Limit" icon="file-zipper">
    **Limit:** 20 MB maximum per message

    Messages exceeding this size will be rejected with an error code. This includes:

    * Email headers
    * Message body (text and HTML)
    * All attachments (base64 encoded)

    <Warning>
      Remember that attachments are base64 encoded, increasing their size by approximately 33%. A 15MB file becomes \~20MB when encoded.
    </Warning>
  </Accordion>

  <Accordion title="Messages Per Connection" icon="envelopes-bulk">
    **Limit:** 10,000 messages maximum per connection

    This is the maximum number of consecutive `MAIL FROM` commands that can be issued for a single SMTP connection.

    When this limit is reached:

    * Additional `MAIL FROM` commands return transient failures
    * You should close and reopen the connection
    * Existing queued messages are not affected

    <Note>
      Most SMTP libraries handle connection pooling automatically, but you may need to configure this for high-volume applications.
    </Note>
  </Accordion>

  <Accordion title="Connection Timeout" icon="clock">
    **Timeout:** 1 minute of inactivity

    Connections are automatically closed if there's no new data within 60 seconds. This applies to:

    * Idle connections between commands
    * Slow data transmission during `DATA` command
    * Unresponsive clients

    <Tip>
      Keep connections active or implement proper reconnection logic in your application to handle timeouts gracefully.
    </Tip>
  </Accordion>
</AccordionGroup>

## SMTP Response Codes

AhaSend returns standard SMTP response codes with specific meanings:

### Success Codes

| Code  | Description                                       |
| ----- | ------------------------------------------------- |
| `250` | **OK** - Message queued successfully for delivery |

### Error Codes

| Code  | Description                                                                                                                                                                                                                         | Action Required                      |
| ----- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------ |
| `421` | **Service Unavailable** - This error could be returned as a result of **disconnecting idle connections** (idle too long), **Load shedding** (server is overwhelmed and cannot accept new connections), or temporary internal errors | Retry with exponential backoff       |
| `451` | **Internal server error** - Temporary server issue                                                                                                                                                                                  | Retry the request                    |
| `500` | **Invalid credentials** - Authentication failed                                                                                                                                                                                     | Check username/password              |
| `521` | **Account disabled** - Account suspended or out of credits                                                                                                                                                                          | Check account status in dashboard    |
| `552` | **Invalid message format** - Malformed email content                                                                                                                                                                                | Validate email structure and headers |
| `553` | **Email rejected** - Recipient domain is toxic, temporary, or has typos                                                                                                                                                             | Verify recipient domain              |
| `556` | **Invalid account or domain** - Account or sending domain not configured                                                                                                                                                            | Verify domain setup                  |

<Warning>
  **553 email Rejection:** This response is only returned if you enable [Recipient Shields](/security/reputation-shield) in your [Account Settings](https://dash.ahasend.com/account/-/settings).
</Warning>

## Special Headers

AhaSend supports special email headers that modify system behavior:

<Card title="Custom Headers" icon="tags" href="/smtp/special-headers">
  Learn about custom headers for tracking, routing, and advanced email features
</Card>

## Example Configurations

### Basic SMTP Configuration

```
Host: send.ahasend.com
Port: 587
Security: STARTTLS
Username: your-smtp-username
Password: your-smtp-password
Authentication: PLAIN
```

### Programming Language Examples

<CardGroup cols={3}>
  <Card title="Node.js" icon="node-js" href="/smtp/nodejs">
    Nodemailer and native SMTP examples
  </Card>

  <Card title="Python" icon="python" href="/smtp/python">
    smtplib and popular framework integrations
  </Card>

  <Card title="PHP" icon="php" href="/smtp/php">
    PHPMailer and framework examples
  </Card>

  <Card title="Go" icon="golang" href="/smtp/golang">
    Native Go SMTP implementations
  </Card>

  <Card title=".NET" icon="microsoft" href="/smtp/dotnet">
    C# and .NET Core examples
  </Card>

  <Card title="Command Line" icon="terminal" href="/smtp/cli">
    sendmail, swaks, and curl examples
  </Card>
</CardGroup>

## Best Practices

<AccordionGroup>
  <Accordion title="Connection Management">
    **Optimize your SMTP connections:**

    * Reuse connections when sending multiple emails
    * Implement connection pooling for high-volume sending
    * Handle timeouts and reconnections gracefully
    * Close connections properly when done
  </Accordion>

  <Accordion title="Error Handling">
    **Handle SMTP errors appropriately:**

    * Implement exponential backoff for `421` responses
    * Retry `451` errors with reasonable delays
    * Don't retry `5xx` errors (permanent failures)
    * Log error codes for debugging and monitoring
  </Accordion>

  <Accordion title="Performance Optimization">
    **Maximize sending performance:**

    * Use persistent connections for batch sending (up to 10k messages per connection)
    * Choose the server closest to your application
    * Monitor connection limits and adjust accordingly
  </Accordion>
</AccordionGroup>

## Security Considerations

<Warning>
  **Always use STARTTLS:** Unencrypted SMTP connections are not secure and may expose your credentials and email content.
</Warning>

<CardGroup cols={2}>
  <Card title="Credential Security" icon="shield-check">
    * Store SMTP credentials securely
    * Use environment variables, not hardcoded values
    * Rotate credentials regularly
    * Use scoped credentials when possible
  </Card>

  <Card title="Domain Authentication" icon="certificate">
    * Verify your sending domains
    * Configure SPF, DKIM, and DMARC records
    * Use dedicated sending domains
    * Monitor domain reputation
  </Card>
</CardGroup>

<Info>
  **Prefer HTTP API?** While SMTP is great for compatibility, our [HTTP API](/send-api/send-email) offers better performance, features, and error handling for modern applications.
</Info>