Hey,
for all of you boys and girls, that had a hard time trying to mail() a multiform/alternative type mail, so that even outlook accepts it as HTML-mail (and not plain-text), here's what finally made it work for me:
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$to = "me@myoffice.de";
$msg = "Hier kommt\r\nauch noch blabla\r\n\r\n--=NextPart_001_01C0AD47.541F6CF0\nContent-Type: text/plain; charset=iso-8859-1\r\nContent-Transfer-Encoding: quoted-printable\r\n\r\n";
$msg.= "This is text part\n\n";
$msg.= "--=NextPart_001_01C0AD47.541F6CF0\nContent-Type: text/html; charset=iso-8859-1\r\nContent-Transfer-Encoding: quoted-printable\r\n\r\n";
$msg.= "<HEAD></HEAD><BODY>This is some HTML</BODY>\r\n\r\n--=NextPart_001_01C0AD47.541F6CF0--";
mail($to,"Test",$msg,"Mime-Version: 1.0\nFrom: $to\nContent-Type: multipart/alternative;\tboundary=\"=NextPart_001_01C0AD47.541F6CF0\"");
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Very important is the '\t' instead of a newline before the boundary information in the mail-header.
Hope, that helps somebody
Tom