The participant has not posted their HTML text. I suspect the problem is that its CID URLs were garbled - I never tested this.
If the Content-ID header is set to this:
Content-Type: image/jpeg
Content-Disposition: inline
Content-ID: afd383988e86ad958709@u
Then the HTML should reference it like this:
<img width="100" height="100" href="cid:afd383988e86ad958709@u" />
, URL- cid "cid:", . (A guid - , , @. , "@u" guid.)
, . Outlook , .
: