THe html email is receiving as html source code not as readable text. WHy?
<html>
<head>
<title></title>
</head>
<body>
<p style="text-align: center; ">
<span style="color:#ff0000;"><span style="font-size:72px;"><span style="background-color:#00ff00;">A test mail</span></span></span></p>
</body>
</html>
Probably because you have sent a plain text email into which you’ve written HTML. There are no shortage of tutorials on writing HTML formatted emails with PHP out there, but make sure you use one that shows you have to write a multipart MIME email rather then plain HTML.