Why does this properly formatted html mail becomes garbled text when forwarded?

[edithsnail]

I'm using Swiftmailer to send the contents of a form to a gmail account. The text appears as good html when first received but it turns into a garbled mix of html code and form contents when forwarded. Why is this? How can it be solved? The message is something like:\[code\]$cuerpo_de_mensaje = "<h1>Resultado de Evaluaci&oacute;n de Curso ". $curso_tomado."</h1>". "<h3>"."Participante"."</h3>". "<p>"."El participante que llen&oacute; la evaluaci&oacute;n fue:&nbsp;"."<b>".$nombre_apellido."</b>"."</p>";\[/code\]And it's sent using:\[code\] $message->setBody( $cuerpo_de_mensaje, 'text/html');\[/code\]Maybe it's this 'text/html' parameter the issue?