Plain text instead of HTML

I looked at the other issue you posted to see what was going on.
Thank you for attaching the raw message including headers to that issue - those hold the key to the problem.
What I see in your message headers is this:

Subject: Conferma richiesta informazioni su SINFI
MIME-Version: 1.0 Content-Type: text/html; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8Bit X-Mailer: Drupal Sender: info@innovazioniperlaterra.org From: "In-TIME S.r.l. - Richiesta informazioni su SINFI"
  <info@innovazioniperlaterra.org> Reply-to: "In-TIME S.r.l. - Richiesta informazioni su SINFI"
  <info@innovazioniperlaterra.org> Cc: info@innovazioniperlaterra.org

You can see that these headers are all wrapped on to one line.
The problem is that recipient email program can only see the "Subject" header and the "MIME-Version" header because headers are recognized only when the appear at the beginning of a line. When the lines are wrapped like this the recipient does not see "Content-Type: text/html" and doesn't know that the body of the email is supposed to be HTML.

The headers in your message SHOULD look like this:

Subject: Conferma richiesta informazioni su SINFI
MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8Bit X-Mailer: Drupal
Sender: info@innovazioniperlaterra.org
From: "In-TIME S.r.l. - Richiesta informazioni su SINFI" <info@innovazioniperlaterra.org>
Reply-to: "In-TIME S.r.l. - Richiesta informazioni su SINFI" <info@innovazioniperlaterra.org>
Cc: info@innovazioniperlaterra.org

Here you can see that each header has its own line, so the recipient can see the "Content-Type: text/html" header and knows that the body is HTML.

This problem is caused by core Drupal, which is why it affected both Webform and Mime Mail. The issue is discussed in 🐛 PhpMail : broken mail headers in PHP 8.0+ because of LF characters Fixed . It has been fixed in core for several weeks, but I haven't tested it yet.

Make sure you are using the latest version of Drupal core, which is 9.5.10, because previous versions do not have this fix.

Many thanks, TR! Your contribution was providential.

Automatically closed - issue fixed for 2 weeks with no activity.