curl -X POST https://api2.ecomailapp.cz/transactional/send-message \
  -H "key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "message": {
      "subject": "Email subject",
      "from_name": "Sender'\''s name",
      "from_email": "foo@bar.cz",
      "reply_to": "foo@bar.cz",
      "text": "Plain text content",
      "html": "<b>Email HTML content</b>",
      "to": [
        {
          "email": "to@me.com",
          "name": "Recipient'\''s name",
          "cc": "cc@me.com",
          "bcc": "bcc@me.com"
        }
      ],
      "global_merge_vars": [
        {"name": "mergeTagName", "content": "mergeTagValue"}
      ],
      "attachments": [
        {
          "type": "mime/type",
          "name": "attachment-name.pdf",
          "content": "base64encodedcontent"
        }
      ],
      "options": {
        "click_tracking": true,
        "open_tracking": true
      }
    }
  }'

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://api2.ecomailapp.cz/transactional/send-message');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'key: YOUR_API_KEY',
    'Content-Type: application/json',
]);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode([
    'message' => [
        'subject' => 'Email subject',
        'from_name' => "Sender's name",
        'from_email' => 'foo@bar.cz',
        'reply_to' => 'foo@bar.cz',
        'text' => 'Plain text content',
        'html' => '<b>Email HTML content</b>',
        'to' => [
            [
                'email' => 'to@me.com',
                'name' => "Recipient's name",
                'cc' => 'cc@me.com',
                'bcc' => 'bcc@me.com',
            ],
        ],
        'global_merge_vars' => [
            ['name' => 'mergeTagName', 'content' => 'mergeTagValue'],
        ],
        'attachments' => [
            [
                'type' => 'mime/type',
                'name' => 'attachment-name.pdf',
                'content' => 'base64encodedcontent',
            ],
        ],
        'options' => [
            'click_tracking' => true,
            'open_tracking' => true,
        ],
    ],
]));
$response = curl_exec($ch);
curl_close($ch);

{
  "results": {
    "total_rejected_recipients": 0,
    "total_accepted_recipients": 2,
    "id": 11668787484950529
  }
}