メールがスパムになるたびにcssとロゴを含むメールを送信すると問題が発生しましたが、cssとロゴを削除すると、受信トレイに正しく配信されます。
以下はコードです
$msgHtml= "<div style='width:650px; font-family:Georgia,Arial, Helvetica, sans-serif; font-size:12px; color:#1f1f1f; background:#f9f9f9; border:1px solid #cfcfcf;'>
<table width='650' border='0' cellspacing='0' cellpadding='4'>
<tr>
<td colspan='4' align='left' valign='top' style='border-bottom:1px solid #cfcfcf;'>
<img src='". base_url() ."assets/user/images/IPIX_logo_mail.jpg' alt='Emcon' alt='' /></td>
</tr>
<td colspan='4' style='height:25px; padding:0;'>
<h3 style='font:bold 14px Georgia, Times New Roman, Times, serif ; color:#333; margin:4px 0 0 0; padding:5px 0px 5px 18px ;'> Hello <span style='font:bold 14px Arial, Helvetica, sans-serif; color:#222222;'>" .$fullname. "</span>,</h3>
</td>
</tr>
<tr>
<td colspan='4' style='height:8px; padding:0px ;margin:0px; border:none;'></td>
</tr>
<tr>
<td colspan='4' style='font:normal 12px Arial, Helvetica, sans-serif; color:#444444; padding:4px 0px 4px 18px;'> Click the link below to reset your password</td>
</tr>
<tr>
<td colspan='4' style='font:normal 12px Arial, Helvetica, sans-serif; color:#444444; padding:4px 0px 4px 18px;'> <a href='".base_url()."home/changeupasswd/".$userid."'>Click Here</a></td>
</tr>
<tr>
<td colspan='4' style='border:none; padding:8px 0 8px 20px; background:rgb(84, 215, 236); color:#fff; font-size:10px; line-height:20px; font:normal 12px Arial, Helvetica, sans-serif;'>© 2012 -13 emcon2013.com All Rights Reserved</td>
</tr>
</table>
</div>";
$sendto = $emailid;
$from = "emailid@test.com";
$subject = "Emcon : Reset password";
$message = $msgHtml;
$header = "From: " . $from . "\n";
$header .= "Content-Type: text/html; charset=iso-8859-1\n";
if (mail($sendto, $subject, $message, $header)) {
echo 1;
} else {
echo 2;
}