I have a multipart/alternative
email that works perfectly on Gmail, Yahoo, and any others I've tried... besides Hotmail (and anything Microsoft I presume.)
The email just appears as raw text on Hotmail.
No matter the amount of times I slam my head against the wall and shout swearwords towards Microsoft which has become a daily activity, I cannot figure out why it doesn't work. Can you?
Here is the email if you want to try it yourself:
Headers:
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_Part_18243133_1346573420.1408991447668"
Body:
------=_Part_18243133_1346573420.1408991447668
Content-Type: text/plain; charset=UTF-8
Hello world.
------=_Part_18243133_1346573420.1408991447668
Content-Type: text/html; charset=UTF-8
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>
<p style="margin-top:50px;font-size:9px;">Hello world</p>
</body></html>
------=_Part_18243133_1346573420.1408991447668--
Here is the full code if you want to test it on your server, either use phpmail or wp_mail() that I'm using.
Update: Here is a source of received message to hotmail.