Tente criar uma mensagem em conformidade com o RFC3156 [0] e enviá-la pelas APIs do Gmail (estou usando a implementação do cliente python).

O que recebo é: na minha caixa do Gmail, a mensagem é enviada corretamente (vejo exatamente a mensagem que criei antes) ·

O que meus receptores recebem é: uma mensagem "multipart / misto" com a mesma carga útil da mensagem que enviei.

Parece que, em algum momento, o Gmail altera meu tipo de conteúdo ... isso é verdade?

Como solução alternativa, estou usando uma conexão SMTP (que age como eu esperava, envia a mensagem corretamente), mas no futuro eu gostaria de evitar essa etapa em favor de um aplicativo de APIs do Gmail puro.

Você pode confirmar ou não esse problema?

[0] - Aqui está um exemplo de teste:

Enviando a mensagem:

message_as_str = '''Content-Type: multipart/signed; protocol="application/pgp-signature";\n micalg="pgp-sha512"; boundary="===============0594182808971957567=="\nMIME-Version: 1.0\nDate: Wed, 12 Nov 2014 12:18:02 +0100\nSubject: A test email\nFrom: Leo Iannacone <[email protected]>\nTo: Andrea Colangelo <[email protected]>\n\n--===============0594182808971957567==\nContent-Type: text/plain; charset="utf-8"\r\nMIME-Version: 1.0\r\nContent-Transfer-Encoding: quoted-printable\r\n\r\nHello,\r\n\r\nthis is a signed email (RFC 3156).\r\n\r\nBest,\r\nL.\r\n\r\n-- =\r\n\r\nUbuntu Member - http://launchpad.net/~l3on\r\nHome Page - http://leoiannacone.com\r\nGPG Key Id - 0xD282FC25\r\n\n--===============0594182808971957567==\nContent-Type: application/pgp-signature; name="signature.asc"\nContent-Description: GooPG digital signature\n\n-----BEGIN PGP SIGNATURE-----\nVersion: GnuPG v1\n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\nuC+56ZJxbKlu73UPsTYh\n=gQKI\n-----END PGP SIGNATURE-----\n\n--===============0594182808971957567==--'''

raw = base64.urlsafe_b64encode(message_as_str)
gmail_service.users().messages().send(userId='me', body={'raw':raw}).execute()

O que tenho na minha caixa de envio é:

Received: from 333313917298-rr7th7jvf4alej59jkv5bbs2fl8sc0j9.apps.googleusercontent.com
    named unknown
    by gmailapi.google.com
    with HTTPREST;
    Wed, 12 Nov 2014 03:23:50 -0800
Content-Type: multipart/signed; protocol="application/pgp-signature";
 micalg="pgp-sha512"; boundary="===============0594182808971957567=="
MIME-Version: 1.0
Date: Wed, 12 Nov 2014 03:23:50 -0800
Subject: A test email
From: Leo Iannacone <[email protected]>
To: Andrea Colangelo <[email protected]>
Message-Id: <CACzqv1fvmbg2Er0Fi-poFSfEDaCLXxW-WGQr8dwHma6t9NjBzA@mail.gmail.com>

--===============0594182808971957567==
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable

Hello,

this is a signed email (RFC 3156).

Best,
L.

-- =

Ubuntu Member - http://launchpad.net/~l3on
Home Page - http://leoiannacone.com
GPG Key Id - 0xD282FC25

--===============0594182808971957567==
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: GooPG digital signature

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iQIcBAABCgAGBQJUY0JcAAoJEDMrlP7SgvwlQakP/jmw6OOcafFAxMnMZNBREnG6
GF3cG/ACJ6BeYhOb0Q2HFQrUKACbsB8QaTxSCf3q7O0HIl5Atr9IVmVIoUuHOM9M
Zt66LWQ3ZUpvyulBswYOq4ERkeeAZh8AYHCz81SUWt42PDn+RrrKESifkgOgy1cM
UpNUXX99cppdY3w1oKVS0SFdhT4ZDNSr6PVUzPzWOHNk3OGOg4okTJOag3lp5R1x
IMQ4plOFNNUs1bDDOpH75TOOGIAdqqgInH5fOk8h2djrPyCozhyllrpCJdiCkrj9
jkL/m0nNS/wHW/pnqSPHZuMDrEYCiiI4fqJYZUkiXQu/koDt40TUqAdeEy1HWJS2
pPqqXjjiZ1jlQ2IBbOG8YfgsLmexTyNE6CmPBXiVk6zY2hnhA6sjq/I9qF8F73SS
FALfkbdRddN7NMzjFVES3eV2gfrUrQnxru2G8urUeeQ2POQrxsGQvb5CrK7VMaUx
teB9UruIpPLNUYR0E7Cr17lqlMR5ILdSlx5xFxIIjg3pfItHQXoOfyusoO1xt8XK
tY8oateT4dgwfR2qrguAnZ96vXtI1soqy8P1TLrJAI/d95kIw4PMzWVm1GjyUiRe
gPhYqvGQV6jc0oUCp+/1MXGUe0zb0meMnBu2f3e/h5Lxkd2z4lPPdxejpNaFc2H1
uC+56ZJxbKlu73UPsTYh
=gQKI
-----END PGP SIGNATURE-----

--===============0594182808971957567==--

O que meu receptor recebe é exatamente:

Delivered-To: [email protected]
Received: by 10.70.104.40 with SMTP id gb8csp207478pdb;
        Wed, 12 Nov 2014 03:23:54 -0800 (PST)
X-Received: by 10.194.94.132 with SMTP id dc4mr64238431wjb.56.1415791433615;
        Wed, 12 Nov 2014 03:23:53 -0800 (PST)
Return-Path: <[email protected]>
Received: from fiordland.canonical.com (fiordland.canonical.com. [91.189.94.145])
        by mx.google.com with ESMTP id 16si39684910wjr.38.2014.11.12.03.23.53
        for <[email protected]>;
        Wed, 12 Nov 2014 03:23:53 -0800 (PST)
Received-SPF: softfail (google.com: domain of transitioning [email protected] does not designate 91.189.94.145 as permitted sender) client-ip=91.189.94.145;
Authentication-Results: mx.google.com;
       spf=softfail (google.com: domain of transitioning [email protected] does not designate 91.189.94.145 as permitted sender) [email protected]
Received: from mail-ig0-f170.google.com (mail-ig0-f170.google.com [209.85.213.170])
    by fiordland.canonical.com (Postfix) with ESMTPS id ABACCA183B1
    for <[email protected]>; Wed, 12 Nov 2014 11:23:52 +0000 (UTC)
Received: by mail-ig0-f170.google.com with SMTP id a13so1193944igq.5
        for <[email protected]>; Wed, 12 Nov 2014 03:23:51 -0800 (PST)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=1e100.net; s=20130820;
        h=mime-version:from:date:message-id:subject:to:content-type;
        bh=W/iDzPadzt4k3bMV8jLK7/5iZLI23kjqCuGNPJ0hdzY=;
        b=MFCFT42IlWfQzbXCCG5slCE4vzpsZg2WQpkmwFa7L46b61Yd/zQ522GgPA1rRhsSrP
         pHhuhPUfCy6HjZRIFf8GqgtTPhinLiuli+ClOz6Oe9rvjH2JK5ILxf0+17woL1n48QAW
         xHnJfo8A/qhBvN42AfdSNVhvcQqQObZ25wBe08zwDJeEWT2XRRiDXcWdWagyqisTronc
         kj3CwRj5AOBThEpzA+wPC0Aalofqqu+YMIMuchBoTsfh437BxkHJLg+il6kDobJXVtXB
         9OPc8o2JqtWwnzHEkC6QKFG54ddoPwvuyq6S325zuDfuKoVrLGeC/kzWRQP+SudBTw0J
         RWBA==
X-Received: by 10.107.135.146 with SMTP id r18mr1699637ioi.62.1415791431765;
 Wed, 12 Nov 2014 03:23:51 -0800 (PST)
Received: from 333313917298-rr7th7jvf4alej59jkv5bbs2fl8sc0j9.apps.googleusercontent.com
 named unknown by gmailapi.google.com with HTTPREST; Wed, 12 Nov 2014 03:23:50 -0800
MIME-Version: 1.0
From: Leo Iannacone <[email protected]>
Date: Wed, 12 Nov 2014 03:23:50 -0800
Message-ID: <CACzqv1fvmbg2Er0Fi-poFSfEDaCLXxW-WGQr8dwHma6t9NjBzA@mail.gmail.com>
Subject: A test email
To: Andrea Colangelo <[email protected]>
Content-Type: multipart/mixed; boundary=001a113f96625df1a80507a7a1d8

--001a113f96625df1a80507a7a1d8
Content-Type: text/plain; charset=ISO-8859-1

Hello,

this is a signed email (RFC 3156).

Best,
L.

-- 
Ubuntu Member - http://launchpad.net/~l3on
Home Page - http://leoiannacone.com
GPG Key Id - 0xD282FC25

--001a113f96625df1a80507a7a1d8
Content-Type: application/pgp-signature; name="signature.asc"
Content-Disposition: attachment; filename="signature.asc"
Content-Transfer-Encoding: base64
X-Attachment-Id: bc758a9561fcd7dc_0.1
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--001a113f96625df1a80507a7a1d8--

Como você pode ver ... O tipo de conteúdo é diferente do que eu tenho nos meus e-mails enviados e do que o meu recebimento entra na caixa de entrada dele.