Emails forwarded to Service Desk email don't come in (when key in envelope-to header)

This is more or less an enhancement of #2247 (closed).

Additional to the already checked headers there is envelope-to which, as my googling skills lead me to, seems used for multiple german providers. See https://www.verbraucherzentrale.de/wissen/digitale-welt/phishingradar/so-lesen-sie-den-mailheader-6077 (German text).

Use case

I setup up support@foo.bar to forward mails to gitlab+myproject-@incoming.foo.bar to set up a small service desk. My mail provider 1&1 IONOS (https://www.ionos.de/) adds the service desk mail to an envelope-to header.

Mail example:

From: "max@winterstein.mx" <max@winterstein.mx>
To: "support@foo.bar" <support@foo.bar>
Subject: test373
Thread-Topic: test373
Thread-Index: AQHVpGT2uiHl62aIf0OT/+w1gFu7uw==
Date: Tue, 26 Nov 2019 14:22:41 +0000
Message-ID: <7e2296f83dbf4de388cbf5f56f52c11f@EXDAG29-1.EXCHANGE.INT>
Accept-Language: de-DE, en-US
Content-Language: de-DE
Content-Type: multipart/alternative;
	boundary="_000_7e2296f83dbf4de388cbf5f56f52c11fEXDAG291EXCHANGEINT_"
MIME-Version: 1.0
Envelope-To: <gitlab+myproject-@incoming.foo.bar>

--_000_7e2296f83dbf4de388cbf5f56f52c11fEXDAG291EXCHANGEINT_
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable



--_000_7e2296f83dbf4de388cbf5f56f52c11fEXDAG291EXCHANGEINT_
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-=
1">
<style type=3D"text/css" style=3D"display:none"><!--P{margin-top:0;margin-b=
ottom:0;} --></style>
</head>
<body dir=3D"ltr" style=3D"font-size:12pt;color:#000000;background-color:#F=
FFFFF;font-family:Calibri,Arial,Helvetica,sans-serif;">
<p><br>
</p>
</body>
</html>

--_000_7e2296f83dbf4de388cbf5f56f52c11fEXDAG291EXCHANGEINT_--

Problem:

Currently gitlab supports only

to
referece
delivered_to

as fields to find the service desk key. This can be easily read from https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/email/receiver.rb .

Solution proposal

I think this could be handled quite easy. Inspired by !1810 (merged) i think a simple solution could be provided by adding a additional function

      def find_key_from_envelope_to_header(mail)
        Array(mail[:envelope-to]).find do |header|
          key = Gitlab::IncomingEmail.key_from_address(header.value)
          break key if key
        end
      end

and enhancing key_from_additional_headers

      def key_from_additional_headers(mail)
        find_key_from_references(mail) ||
          find_key_from_delivered_to_header(mail) ||
          find_key_from_envelope_to_header(mail) 
      end

Edited Jan 27, 2020 by Max Winterstein