Integrate EffiLink with Your App via SMTP

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:

smtp.effilink.co

Hong Kong China:

hk-smtp.effilink.co

European Union:

eu-smtp.effilink.co

2、Use the following settings in your SMTP client or application: Username & Password: You may use any active API key associated with your account as the SMTP password. As the API key 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

You may also need a sending domain (for example, mail.example.com). See sender domain management for details.

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.

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
`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.

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).

X-Easeye-ExpirationDate: 2024-10-09 11:03:25

Full example with `X-Easeye-*` headers

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>

Get Started

Authentication

Email

SMS

Contacts & Lists

Webhooks

Limits & Quotas

Integrate EffiLink with Your App via SMTP

Connection settings

SMTP response codes

SMTP extensions with `X-Easeye-*` headers

`X-Easeye-API`

`X-Easeye-UniqueMsgID`

`X-Easeye-ExpirationDate`

Full example with `X-Easeye-*` headers

Security checklist

​Connection settings

​SMTP response codes

​SMTP extensions with X-Easeye-* headers

​X-Easeye-API

​X-Easeye-UniqueMsgID

​X-Easeye-ExpirationDate

​Full example with X-Easeye-* headers

​Security checklist

Connection settings

SMTP response codes

SMTP extensions with `X-Easeye-*` headers

`X-Easeye-API`

`X-Easeye-UniqueMsgID`

`X-Easeye-ExpirationDate`

Full example with `X-Easeye-*` headers

Security checklist