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

# Integrate EffiLink with Your App via SMTP

> Connect any SMTP-compatible app or library to EffiLink using your API key as a password to send transactional or marketing emails reliably.

EffiLink exposes a standard SMTP interface so you can send emails from any application, framework, or email library that supports SMTP — without modifying your existing mail-sending code. Simply point your SMTP client at EffiLink's servers, authenticate with your API key, and your emails are routed through EffiLink's delivery infrastructure.

SMTP is ideal for legacy applications, CMS platforms, and any environment where switching to the REST API is not practical.

***

## Connection settings

**1 、Choose the endpoint closest to your region:**

Mainland China:

```text theme={null}
smtp.effilink.co
```

Hong Kong China:

```text theme={null}
hk-smtp.effilink.co
```

European Union:

```text theme={null}
eu-smtp.effilink.co
```

**2、Use the following settings in your SMTP client or application:**

**Username & Password:**

You may use any active <kbd>API key</kbd> associated with your account as the SMTP password. As the <kbd>API key</kbd> alone is sufficient for authentication, EffiLink does not rely on the SMTP username for request authentication. We recommend using your EffiLink account’s primary contact email address as the username.

**Ports:**

* **Un-encrypted connection**: Use ports 1026 or 1027
* **Encrypted connection (SSL/TLS):** Use port 4026 or 4027

<Tip>
  You may also need a sending domain (for example, `mail.example.com`). See sender domain management for details.
</Tip>

***

## SMTP response codes

Every SMTP call returns a response code:

* **2xx — Success:** The email has been accepted for delivery.
* **4xx — Temporary failure:** A transient error occurred. The message can be retried later.
* **5xx — Permanent failure:** A permanent error occurred. The message should not be retried.

| Code  | Message                                                          | Cause                                           | Resolution                                                     |
| ----- | ---------------------------------------------------------------- | ----------------------------------------------- | -------------------------------------------------------------- |
| `250` | `ok data code message [txsID=...]; in reply to DATA`             | Server accepted the message                     | No action needed                                               |
| `530` | `Authentication failed`                                          | Wrong API key, wrong secret, or expired account | Verify your API key in the EffiLink dashboard                  |
| `540` | `permission required`                                            | Feature not enabled on your account             | Contact EffiLink support to enable the feature                 |
| `553` | `mail from <x> not allowed`                                      | Sender not registered on the platform           | Register and verify the sender in your EffiLink account        |
| `553` | `mail <x> not allowed (mailbox syntax incorrect)`                | Recipient address has invalid format            | Check the recipient address (see recipient format rules below) |
| `502` | `Email content is too large ([length])`                          | Email exceeds the 10 MB size limit              | Reduce attachment size or host large files externally          |
| `452` | `Insufficient points error (accountId=..., availablePoints=...)` | Account balance too low to send                 | Contact EffiLink support to top up your account                |

## SMTP extensions with `X-Easeye-*` headers

EffiLink supports custom SMTP headers that give you access to platform features — tracking, categorization, priority, and message expiry — without leaving your SMTP workflow.

### `X-Easeye-API`

Pass a JSON payload to configure tracking, categorization, campaign labeling, priority, and custom tracking domains.

```text theme={null}
X-Easeye-API: {"category":"Registration","campaign":"registration email for new users","send_options":{"track_open":1,"track_click":1,"track_subscription":1,"custom_domain":"http://linktrace.test.com"},"priority":"low"}
```

| Field                             | Type   | Description                                                                                                                         |
| --------------------------------- | ------ | ----------------------------------------------------------------------------------------------------------------------------------- |
| `category`                        | string | Email category (*e.g. Registration*). ***Max length***\*:\* 100 bytes. ***Allowed characters***: A‑Z, a‑z, 0‑9                      |
| `campaign`                        | string | Email task name. ***Max length***: 100 bytes. ***Allowed characters***: A‑Z, a‑z, 0‑9.                                              |
| `send_options.track_open`         | int    | `1`  enable open tracking                                                                                                           |
| `send_options.track_click`        | int    | `1`  enable click tracking.  ***Specific URLs*** can be excluded from tracking if needed,See [Email Tracking](/docs/email-tracking) |
| `send_options.track_subscription` | int    | Whether to add ***unsubscribe link***. Add when value is `1`; do not add for other values.                                          |
| `send_options.custom_domain`      | string | Custom domain used for tracking email links                                                                                         |
| `send_options.priority`           | string | `high`, `median`, or `low` (default: `low`)                                                                                         |

### `X-Easeye-UniqueMsgID`

A unique identifier for the message, used for deduplication and tracking. Maximum 50 bytes; alphanumeric characters and dashes only.

```text theme={null}
X-Easeye-UniqueMsgID: order-confirm-1234-abc
```

### `X-Easeye-ExpirationDate`

Set an expiry time for the message. If the email has not been delivered by this time, EffiLink will stop attempting delivery and return an expired status. Format: `yyyy-MM-dd HH:mm:ss` in UTC+8 (China Standard Time).

```text theme={null}
X-Easeye-ExpirationDate: 2024-10-09 11:03:25
```

### Full example with `X-Easeye-*` headers

```shellscript theme={null}
EHLO yourdomain.com
AUTH LOGIN
[base64 username]
[base64 API key]
MAIL FROM:<no-reply@yourdomain.com>
RCPT TO:<user@example.com>
DATA
From: Your App <no-reply@yourdomain.com>
To: user@example.com
Subject: Welcome to EffiLink
X-Easeye-API: {"category":"Onboarding","send_options":{"track_open":1,"track_click":1},"priority":"low"}
X-Easeye-UniqueMsgID: welcome-user-5678
Content-Type: text/html; charset=UTF-8

<p>Welcome! Thanks for signing up.</p>
```

***

## Security checklist