OK, I know it's generally considered a bad idea to send HTML emails, but a client wants it.

I've figured out how to send to Hotmail and get my images show, but not as attachments, using Mimetype (Mimes!), but they come out all wrong in Outlook - is there a special trick for doing this bad idea? Thanks.

Right, well it finally works, thanks. I've got no idea why it wasn't before - for all intents and purposes it was exactly the same, just put together slightly differently. Are headers really that fickle?

Try this...
This works both in thunderbird and outlook for me. At least if this doesn't work you can put it down to your client with more certainty.

$textemail="This is the text part of the message.\n";
$htmlemail="

Header

Paragraph

\n";
$subject="Test Email";
$to="Test ";
$from="HTML Tester ";
$mime_boundary="==Multipart_Boundary_sjs48643ik";
$headers = "From: $from\n" .
"Reply-to: $from\n".
"X-Mailer: PHP Mail\n".
"MIME-Version: 1.0\n" .
"Content-Type:multipart/alternative;\n" .
" boundary=\"".$mime_boundary."\"\n";

$message = "This is a multi-part message in MIME format.\n\n" .
"--".$mime_boundary."\n" .
"Content-Type: text/plain; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
$textemail .
"--".$mime_boundary."\n" .
"Content-Type: text/html; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
$htmlemail;
if (mail($to, $subject, $message, $headers))
echo "Email Sent!";
else
echo "Failed to send message.";

I'm sure this bit is the problem:

Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: 8bit

They are the headers to the plain text part, and show up in Outlook with the plain text message. The boundaries blatantly work because they successfully split the message up into it's alternative parts.

*POP*

This is really annoying now, as it'll start holding up a couple of projects if it carries on much longer. The message source is fine, and as far as I can tell the rest of it is fine. But it refuses to work.

BUT!!!

I've had HTML emails forwarded to me that have worked in someone else's Outlook, and the headers, boundaries, etc. work in exactly the same way, but these messages haven't displayed as HTML in my Outlook either. It's set to HTML. Hate to admit it, but I'm stumped.

From looking at the mail source in Outlook, it's all in place as it should be. Just thought of something else I could try...

EDIT: Nah, still nothing doing. I built the headers up as you suggested, but it looks exactly the same in the message source. The problem lies when it's detecting the boundary for the plain text, but it's no different from the countless other examples I've seen.

Well, I tried copy & pasting the code and sending a mail with it, certainly didnt give me HTML emails in thunderbird but thats possibly because its lost something in the copy & paste.

What I'd be tempted to do especially with the headers is rather than relying on 1 big multi-line string to put it all the information together is build it up line by line yourself and making sure you've got the correct number of linefeeds and spaces. eg

$headers = "From: $from\n";
$headers.= "MIME-Version: 1.0\n";

..and so on. Email clients are generally very unforgiving when it comes to parsing emails, an extra "\n" or a "\r" where it shouldn't be and it'll fall to pieces.

It could well be your email client thats the problem here too, but I guess that theory is easily tested.

Looks a bit wrong there because of the line length, but it sends absolutely fine, works perfect in Hotmail, works perfect as just HTML in Outlook, and I've removed all remnants of the company.

$message = "This is a multi-part message in MIME format.

--==MXCVS_73849576_REGIONFREE_NANU-NANU==
Content-Type: text/plain; charset=" . '"' . "iso-8859-1" . '"' . "
Content-Transfer-Encoding: 7bit

plain text content

--==MXCVS_73849576_REGIONFREE_NANU-NANU==
Content-Type: text/html; charset=" . '"' . "iso-8859-1" . '"' . "
Content-Transfer-Encoding: 7bit

HTML text content

--==MXCVS_73849576_REGIONFREE_NANU-NANU==--"

mail("$this_email", "$subject", "$message",
"Date: $page_date, $page_time \n" .
"Return-Path: Newsletter \n" .
"From: Newsletter \n" .
"Organization: Muh \n" .
"X-Mailer: PHP 4.3.11 \n" .
"MIME-Version: 1.0 \n" .
"Content-Type: multipart/alternative; boundary=" . '"' . "==MXCVS_73849576_REGIONFREE_NANU-NANU==" . '"' . " \n\n");

You on MSN Garin?

I'm sure it's not the code. Although that means that it probably is.

All I can suggest is...show us some code.