Guide How to Use the BillionMail Send API for Transactional Email (With curl and PHP)

Guide How to Use the BillionMail Send API for Transactional Email (With curl and PHP) - Printable Version

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How to Use the BillionMail Send API for Transactional Email (With curl and PHP) - JAY - 10-23-2026

How to Use the BillionMail Send API for Transactional Email (With curl and PHP)

Newsletters are only half of what most sites send. The other half is transactional email: password resets, order receipts, sign-up confirmations and login codes. These go to one person, right now, because they did something. BillionMail has a built-in Send API for exactly this, so your app or website can send through your own mail server instead of a paid service.

This guide shows how to create an API, call it with curl and PHP, and put personal values like a name or a code into the email.

Before you start

BillionMail is installed and your domain's DNS is green. If not, start with installing BillionMail and the DNS setup guide.
You have a template for the email. A simple HTML template is fine; see BillionMail email templates.
Outgoing port 25 works on your server, or you use a relay. See how to check if port 25 is blocked.

Step 1: Create the API
Open Send API in the left menu and click Create API. The form looks like this:
[Image: attachment.php?aid=428]

Fill it in like this:

API Name: something you'll recognise later, for example "Password reset".
From: pick your domain and the mailbox it sends from, such as
Code:
[email protected]
.
Display Name: the name people see in their inbox, like "Example Support".
Subject and Email template: the subject line and the template to use.
Status: on.

Under Advanced Features you'll find Unsubscribe link, Track Clicks, Track Opens, IP Whitelist and Recipient Groups. For transactional mail, turn the unsubscribe link off (people can't unsubscribe from a password reset) and keep tracking off unless you need it. Tracking rewrites links, and some spam filters don't like that in security emails.

IP Whitelist is worth filling in: enter your app server's IP address (several are separated by commas). Then the key only works from that server, even if it leaks. Empty means any IP can use it.

After you save, the API appears in the list with its API Key, plus send count and open, click and bounce rates.

Step 2: Send a test with curl
The endpoint is

Code:

/api/batch_mail/api/send

on your BillionMail address (the same address and port you use for the dashboard). Run this from your app server:

Code:

curl -X POST 'https://mail.example.com/api/batch_mail/api/send' \
  -H 'X-API-Key: YOUR_API_KEY' \
  -H 'Content-Type: application/json' \
  -d '{"recipient":"[email protected]"}'

A good answer has

Code:

"success":true

and

Code:

"code":0

. If you get a certificate error because you're testing with an IP or a self-signed certificate, fix the certificate rather than adding

Code:

-k

in production; see BillionMail SSL fixes.

Step 3: Put personal values in the email
Most transactional emails need something personal: a name, a reset link, a code. Send them in

Code:

attribs

Code:

{"recipient":"[email protected]",
 "attribs":{"name":"Jane","code":"482913","link":"https://example.com/reset?t=abc"}}

In the template, each value is written as

Code:

{{.API.name}}

Code:

{{.API.code}}

and so on. For example:

Code:

<p>Hi {{.API.name}},</p>
<p>Your login code is <b>{{.API.code}}</b>. It expires in 10 minutes.</p>

The names are case-sensitive:

Code:

code

in the request means

Code:

{{.API.code}}

in the template, not

Code:

{{.API.Code}}

.

Step 4: Call it from PHP

Code:

<?php
function send_mail($to, array $vars) {
    $ch = curl_init('https://mail.example.com/api/batch_mail/api/send');
    curl_setopt_array($ch, [
        CURLOPT_POST => true,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_TIMEOUT => 15,
        CURLOPT_HTTPHEADER => ['X-API-Key: ' . getenv('BM_API_KEY'), 'Content-Type: application/json'],
        CURLOPT_POSTFIELDS => json_encode(['recipient' => $to, 'attribs' => $vars]),
    ]);
    $res = json_decode(curl_exec($ch), true);
    curl_close($ch);
    return !empty($res['success']);
}
send_mail('[email protected]', ['name' => 'Jane', 'code' => '482913']);

Keep the key in an environment variable or a config file outside the web root, never in JavaScript that runs in the browser.

Sending to several people
For a small list, use

Code:

/api/batch_mail/api/batch_send

with

Code:

"recipients":["[email protected]","[email protected]"]

instead of

Code:

recipient

. For real newsletters, use a campaign instead; see send your first newsletter.

If it doesn't work

401 or "invalid key": check the header name is exactly
Code:
X-API-Key
and the API's status is on.
Forbidden from your server: the IP Whitelist doesn't include the IP your app really uses. Check with
Code:
curl -4 ifconfig.me
on the app server.
Success, but nothing arrives: the email is in the queue. See emails stuck in the send queue.
Variables show up empty: the name in
Code:
attribs
doesn't match the template exactly.

More BillionMail guides

Applies to BillionMail 4.9, October 2026. Replace

Code:

example.com

and

Code:

mail.example.com

with your own domain and mail hostname. Menu names can change a little between versions.