Issue symptoms
A requester sends an email with HTML elements, and they display as <b>example</b> on a ticket comment, rather than HTML formatted text: example.
Resolution steps
The original source of the incoming email includes a Content-Type: text/plain in the header and email declaration block. For this reason, the content type that the email displays is plain text rather than an HTML one. This is not a Zendesk-specific issue and needs to be corrected on the sender’s end.
Advise the original email sender to reach out to their email administrator for them to replace Content-Type: text/plain with Content-Type: text/HTML from their email header. This will allow HTML format to render on the receiving end.
For more information, see this article: Disabling and enabling rich content in incoming emails.