[SOLVED] Google (inbox, gmail, now) doesn't recognize EventReservation

I'm adding EventReservation JSON-LD into email (sent from me to me using gmail API from Chrome Extension).

It is successfully delivered into my inbox, but google doesn't recognize this as an event. Here it is email body dump from delivered email (HTML part is successfully validated by google structured data testing tool and yandex structured data validator, and actually json is copy-pasted from an real event that was parsed in my Inbox, so script should be good):

Received: from 236444161893
    named unknown
    by gmailapi.google.com
    with HTTPREST;
    Sat, 26 Mar 2016 07:34:52 -0400
From: XXXXXXX@gmail.com
To: XXXXXXX@gmail.com
Subject: Lets Celebrate !
Content-Type: text/html; charset="utf-8"
Date: Sat, 26 Mar 2016 07:34:52 -0400
Message-Id: <CAEY7X7rQZFQTmYqyH-UEMt5KR3bwbC5D-KhT3Rv8vKixVrGkFA@mail.gmail.com>


<html>
    <head>
        <title>Sample Title</title>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    </head>
    <body>
        <script type="application/ld+json">
{
    "@context": "http://schema.org",
    "@type": "EventReservation",
    "reservationNumber": "123123123",
    "underName": {
        "@type": "Person",
        "name": "John Smith"
    },
    "reservationStatus": "http://schema.org/Confirmed",
    "reservationFor": {
        "@type": "Event",
        "name": "ProductCamp Helsinki 2016",
        "startDate": "2016-04-09T10:00:00+03:00",
        "endDate": "2016-04-09T16:00:00+03:00",
        "location": {
            "@type": "Place",
            "name": "OP Headquarters",
            "address": {
                "@type": "PostalAddress",
                "streetAddress": "Teollisuuskatu 1E00510 Helsinki",
                "addressLocality": "Helsinki",
                "addressRegion": "",
                "postalCode": "00510",
                "addressCountry": "FI"
            }
        }
    }
}
        </script>
        <b>I did it!</b>
    </body>
</html>

EventReservation Email in my Inbox

What could be wrong?

UPDATE: This HTML works and parsed by Inbox when I send it through script.google.com (MailApp.sendEmail), but doesn't work if it is sent from Chrome Extension using gmail API (POST https://www.googleapis.com/gmail/v1/users/me/messages/send) . So it looks like something wrong with email headers. E.g. "correct" email sent from script.google.com contains DKIM-Signature etc:

Received-SPF: pass (google.com: domain of 3JIP2VgkJCcEiluklyzwinthps.jvtiluklyzwinthps.jvt@maestro.bounces.google.com designates 2607:f8b0:400c:c05::245 as permitted sender) client-ip=2607:f8b0:400c:c05::245;
MIME-Version: 1.0
Message-ID: <001a1146937894fe1e052ef2fa6a@google.com>
Date: Sat, 26 Mar 2016 12:40:04 +0000
Subject: Test Email markup - Sat Mar 26 2016 15:40:04 GMT+0300 (EAT)
From: XXXXXXX@gmail.com
To: XXXXXXX@gmail.com
Content-Type: multipart/alternative; boundary=001a1146937894fe08052ef2fa67

--001a1146937894fe08052ef2fa67
Content-Type: text/plain; charset=ISO-8859-1; format=flowed; delsp=yes

I did it!

--001a1146937894fe08052ef2fa67
Content-Type: text/html; charset=ISO-8859-1

<html>
    <head>
        <title>Sample Title</title>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    </head>
    <body>
        <script type="application/ld+json">
{
    "@context": "http://schema.org",
    "@type": "EventReservation",
    "reservationNumber": "1231231234",
    "underName": {
        "@type": "Person",
        "name": "John Smith"
    },
    "reservationStatus": "http://schema.org/Confirmed",
    "reservationFor": {
        "@type": "Event",
        "name": "ProductCamp Helsinki 2016",
        "startDate": "2016-04-09T10:00:00+03:00",
        "endDate": "2016-04-09T16:00:00+03:00",
        "location": {
            "@type": "Place",
            "name": "OP Headquarters",
            "address": {
                "@type": "PostalAddress",
                "streetAddress": "Teollisuuskatu 1E00510 Helsinki",
                "addressLocality": "Helsinki",
                "addressRegion": "",
                "postalCode": "00510",
                "addressCountry": "FI"
            }
        }
    }
}
        </script>
        <b>I did it!</b>
    </body>
</html>
--001a1146937894fe08052ef2fa67--