nextcloud
Embed images when sending HTML E-Mail
external resources are not automatically loaded in sanely configured E-Mail clients.
For example in Thunderbird a warning like this will pop up:
the NextCloud logo is fetched from https://yourserverhere.com/core/img/logo.png?v=3D0 which is opens the door for resource based tracking.
It's not just the NextCloud logo but all image resources. For example in an mail based invite the small icons like 
and 
will not be displayed when viewed in a many E-Mail client (for example Thunderbird with default settings)
Why change this?
- privacy (readers can be sure that the NC admin does not track them)
- usability (readers can see the images without clicking away a warning prompt)
Is this technically possible?
- Yes, you can embed images. For example via MIME Content-ID or via inline embedding
Technical Example
Black and white image reading "Hello" is inserted into an HTML E-Mail via Content-ID. The image is base64 encoded inside the E-Mail
Return-path: <[email protected]>
Received: ...
Original-recipient: rfc822;[email protected]
Sender: [email protected]
To: Me <[email protected]>
From: Me <[email protected]>
Subject: Test
Message-id: <[email protected]>
Date: Sun, 13 Jan 2019 00:00:00 +0000
MIME-version: 1.0
Content-type: multipart/alternative;
boundary=------------63E36A7002E8D6FBEB9C5425
Content-language: en-US
This is a multi-part message in MIME format.
--------------63E36A7002E8D6FBEB9C5425
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
--------------63E36A7002E8D6FBEB9C5425
Content-Type: multipart/related;
boundary="------------885E82E985699C9CB72624B8"
--------------885E82E985699C9CB72624B8
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 7bit
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<img src="cid:[email protected]" alt="">
</body>
</html>
--------------885E82E985699C9CB72624B8
Content-Type: image/png;
name="poclpdkelpocpgpa.png"
Content-Transfer-Encoding: base64
Content-ID: <[email protected]>
Content-Disposition: inline;
filename="poclpdkelpocpgpa.png"
iVBORw0KGgoAAAANSUhEUgAAADgAAAASCAIAAACB04oZAAAAzUlEQVRIie2T0REDIQhEqYuC
qIdqaGaLMR9Rz1PGU+NMLpPbP0HXByKFHxF9G2BU/whqQlGsmMxeqwTNXqwIAcpxKTbjCOUO
Sj87CBptysCC7S1ATVKXiZjFvBsXQaGHN0u7ZQIUyiyGvDRldqZiBdTkHIVJbd2AUqVsYNIO
qxdbAHViTcuGO+rUEAv5GNQrtwmOPz08u0GUi+zWjoY0SYcjTHnL09fOQDv+VO4+zWXx1vkM
TMt/L3qY+aORTvaz7x0zv/7GekB36wHdrRcZX4h7tVmUbQAAAABJRU5ErkJggg==
--------------885E82E985699C9CB72624B8--
--------------63E36A7002E8D6FBEB9C5425--
@JavaZauber @pirate @mokken would you like to contribute a fix for this? Or do you have any pointers what is necessary to fix this? :)
Easier alternative for developer: Not using images at all. Composition seems to be implemented with IEMailTemplate.php
Unfortunately when I looked at this awhile back I found inline embeds to be an obstacle if one care's about recipients using, say, Gmail. I don't recall finding a good path forward so that effort stalled out.
Similar to: https://github.com/nextcloud/server/issues/17187#issuecomment-692551296
