Hi fliebe92,
thanks for getting back to me!
Your reply made me curious, if it really is my signature that is breaking the html.
I tested by disabling the signature, sending an email - Works as expected. Good! 
I sent another email, signature still disabled, but copied an image into the text field: Broken.
It is not our signature that is breaking things.
At this point I think this is a bug report, and I’d ask you to investigate or try to reproduce this on a test environment.
I will recap the important bits of information:
Currently, we are using the application_wrapper.html.erb that you linked, we have not modified this file.
Our ###html_email_css_font### setting is attached as a screenshot.
This is the html source that we receive when we send emails WITHOUT images embedded:
<!DOCTYPE html><html dir="auto"><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<style type="text/css">
body {
font-family:Calibri;font-size:11pt;
}
img {
outline: none;
text-decoration: none;
-ms-interpolation-mode: bicubic;
}
a img {
border: none;
}
table td {
border-collapse: collapse;
}
table {
border-collapse: collapse;
mso-table-lspace: 0pt;
mso-table-rspace: 0pt;
border: none;
table-layout: auto;
display: block;
width: 100%;
overflow: auto;
word-break: keep-all;
}
table,
pre,
blockquote {
margin: 0 0 16px;
}
td, th {
padding: 7px 12px;
}
th {
font-weight: bold;
text-align: center;
}
col {
width: auto;
}
p {
margin: 0;
}
code {
border: none;
background: hsl(0,0%,97%);
white-space: pre-wrap;
}
blockquote {
padding: 8px 12px;
border-left: 5px solid #eee;
}
pre {
padding: 12px 15px;
font-size: 13px;
line-height: 1.45;
background: hsl(0,0%,97%);
white-space: pre-wrap;
border-radius: 3px;
border: none;
overflow: auto;
}
</style>
</head>
<body style="font-family:Calibri;font-size:11pt;;">
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
</body>
</html>
And this is the html-source we receive when we send emails WITH images embedded. (I also tested with different images and fresh stock images, so no, our specific images are not causing this):
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"><body>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<style type="text/css">
body {
font-family:Calibri;font-size:11pt;
}
img {
outline: none;
text-decoration: none;
-ms-interpolation-mode: bicubic;
}
a img {
border: none;
}
table td {
border-collapse: collapse;
}
table {
border-collapse: collapse;
mso-table-lspace: 0pt;
mso-table-rspace: 0pt;
border: none;
table-layout: auto;
display: block;
width: 100%;
overflow: auto;
word-break: keep-all;
}
table,
pre,
blockquote {
margin: 0 0 16px;
}
td, th {
padding: 7px 12px;
}
th {
font-weight: bold;
text-align: center;
}
col {
width: auto;
}
p {
margin: 0;
}
code {
border: none;
background: hsl(0,0%,97%);
white-space: pre-wrap;
}
blockquote {
padding: 8px 12px;
border-left: 5px solid #eee;
}
pre {
padding: 12px 15px;
font-size: 13px;
line-height: 1.45;
background: hsl(0,0%,97%);
white-space: pre-wrap;
border-radius: 3px;
border: none;
overflow: auto;
}
</style>
<div>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. </div>
<div>
<img style="width: 241px; max-width: 100%;" src="cid:14167.e78863b4-e982-4403-8e85-c3196de7bdbd@zammad.od.local" width="241"><br>
</div>
<div>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.<br>
</div>
</body>
The second output suddenly has an opening body-tag right after the first meta tag. As this is still within the html-head, of course this is breaking the html/css settings defined afterwards from taking effect. Why including an image would cause this is beyond me.
Again, we use Debian 11 (I mistyped in my first post, sorry!)
With Zammad v 6.3.1, installed via package.
Thank you for the patience you have with me.
