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: <user@example.com>
Received: ...
Original-recipient: rfc822;user@example.com
Sender: user@example.com
To: Me <user@example.com>
From: Me <user@example.com>
Subject: Test
Message-id: <123e4567-e89b-12d3-a456-426655440000@example.com>
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:part1.1389FB42.0D8EC5D1@example.com" alt="">
</body>
</html>
--------------885E82E985699C9CB72624B8
Content-Type: image/png;
name="poclpdkelpocpgpa.png"
Content-Transfer-Encoding: base64
Content-ID: <part1.1389FB42.0D8EC5D1@example.com>
Content-Disposition: inline;
filename="poclpdkelpocpgpa.png"
iVBORw0KGgoAAAANSUhEUgAAADgAAAASCAIAAACB04oZAAAAzUlEQVRIie2T0REDIQhEqYuC
qIdqaGaLMR9Rz1PGU+NMLpPbP0HXByKFHxF9G2BU/whqQlGsmMxeqwTNXqwIAcpxKTbjCOUO
Sj87CBptysCC7S1ATVKXiZjFvBsXQaGHN0u7ZQIUyiyGvDRldqZiBdTkHIVJbd2AUqVsYNIO
qxdbAHViTcuGO+rUEAv5GNQrtwmOPz08u0GUi+zWjoY0SYcjTHnL09fOQDv+VO4+zWXx1vkM
TMt/L3qY+aORTvaz7x0zv/7GekB36wHdrRcZX4h7tVmUbQAAAABJRU5ErkJggg==
--------------885E82E985699C9CB72624B8--
--------------63E36A7002E8D6FBEB9C5425--
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=3D0which 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?
Is this technically possible?
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