You must include MIME-Version as well as Content-Type, for example this will work, assuming every line ends with "\r\n".
MIME-Version: 1.0
Date: Thu, 16 Jun 2022 14:12:05 +0000
To: Test <test@gmail.com>
From: Info <info@test.business>
Message-ID: <5083142354314465839@something>
Subject: Post Test 1
Content-Type: text/html; charset="UTF-8"
<h1>Header</h1><b>Bold</b><br><div style="color:#f00">Red text</div>