Why an embedded favicon can broke an email server?

Question

I have sent an email to a colleague with the following html code (a embedded favicon) pasted in Thunderbird:

<link rel="shortcut icon" href="data:image/vnd.microsoft.icon;base64,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"/>

Then his Outlook stop receiving emails with an error 0x800CCC0F (server connection problem) and it doesn't receive the "poison" email. So I though is the problem of the mail server.

But my question is, why a "simple" text can broke an email server?

---

This is the main part of the message sent:

Content-Type: multipart/alternative;
 boundary="------------040701090409010309050003"

This is a multi-part message in MIME format.
--------------040701090409010309050003
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: quoted-printable

<link rel=3D"shortcut icon"=20
href=3D"data:image/vnd.microsoft.icon;base64,AAABAAEAEBAAAAEAIABoBAAAFgAA=
ACgAAAAQAAAAIAAAAAEAIAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAB+s8sQfrPL/H6zyxAAAAAAfrPL/H6zy/wAAAAAfrPL/H6zyKh+s=
8v8AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH6zy/wAAAAAAAAAAH6zy/x+s8k8AAAAAA=
AAAAB+s8v8AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH6zyxB+s8v8frPLEAAAAAB+s8v=
8frPL/AAAAAB+s8v8frPIqH6zy/wAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
B+s8v8frPL/H6zy/x+s8v8AAAAAH6zy/wAAAAAAAAAAH6zy/wAAAAAfrPL/H6zy/x+s8v8frP=
L/AAAAAAAAAAAfrPL/AAAAAAAAAAAfrPL/AAAAAB+s8v8AAAAAAAAAAB+s8v8AAAAAH6zy/wA=
AAAAAAAAAAAAAAAAAAAAAAAAAH6zy/wAAAAAfrPL/H6zy/wAAAAAfrPL/AAAAAAAAAAAfrPL/=
AAAAAB+s8v8AAAAAAAAAAAAAAAAAAAAAAAAAAB+s8v8AAAAAAAAAAAAAAAAAAAAAH6zy/x+s8=
v8frPL/H6zy/wAAAAAfrPL/AAAAAAAAAAAAAAAAAAAAAAAAAAAfrPL/AAAAAAAAAAAAAAAAAA=
AAAB+s8v8AAAAAAAAAAB+s8v8AAAAAH6zy/wAAAAAAAAAAAAAAAAAAAAAAAAAAH6zy/x+s8v8=
frPL/H6zy/wAAAAAfrPL/AAAAAAAAAAAfrPL/AAAAAB+s8v8frPL/H6zy/x+s8v8AAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA//8AAP//AADiVwAA9u8AAOJXAAD//wAA//8AAIWh=
AAC1rwAApa8AALwvAAC9rwAAhaEAAP//AAD//wAA//8AAA=3D=3D"/>

--------------040701090409010309050003
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>
    &lt;link rel="shortcut icon"
href=<a class="moz-txt-link-rfc2396E" href="data:image/vnd.microsoft.icon;base64,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">"data:image/vnd.microsoft.icon;base64,AAABAAEAEBAAAAEAIABoBAAAFgAAACgAAAAQAAAAIAAAAAEAIAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB+s8sQfrPL/H6zyxAAAAAAfrPL/H6zy/wAAAAAfrPL/H6zyKh+s8v8AAAAAAAAAAAAAAAAAA
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
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA//8AAP//AADiVwAA9u8AAOJXAAD//wAA//8AAIWhAAC1rwAApa8AALwvAAC9rwAAhaEAAP//AAD//wAA//8AAA=="</a>/&gt;<br>
  </body>
</html>

--------------040701090409010309050003--