[Phpmailer-general] Newbie: Filesize > 1.5MB = "Could not instantiate function"

[James B. <stu...@gm...>]

Windows IIS7
PHP v.5.4
PHPMailer v.5.2.2

Sending messages with attachments < 1.5MB works just fine.

Sending message with attachment > 1.5MB produces error:

"Could not instantiate mail function."

File IS uploaded and available for attachment.

Here's an expanded error message:

start==============
phpMailerException: *Could not instantiate mail function.*

print_r(error_get_last()) used to get:

Array ( [type] => 2 [message] => mail(): Failed to Send [file] =>
\Path\to\PHPMailer\class.phpmailer.php [line] => 447 )

File info: uploads/test.docx : size: 2333.6083984375
end==============

Here is class.phpmailer.php lines 443-450:

start==============
  private function mail_passthru($to, $subject, $body, $header, $params) {
    if ( ini_get('safe_mode') || !($this->UseSendmailOptions) ) {
        $rt = @mail($to,
$this->EncodeHeader($this->SecureHeader($subject)), $body, $header);
    } else {
447=>        $rt = @mail($to,
$this->EncodeHeader($this->SecureHeader($subject)), $body, $header,
$params);
    }
    return $rt;
  }
end==============

My php.ini specifies (and phpinfo() shows) both post_max_size and
upload_max_filesize = 20M

My upload form contains MAX_FILE_SIZE of 41943040

I have included ini_set commands to extend the execution and input times of
the script to 25 minutes (the send process takes less than 3 seconds even
with a big attachment).

I have tried using mb_encode_mimeheader() instead of
EncodeHeader(..SecureHeader()..) as suggested on the web.

Almost all of the suggestions I have found online talk about updating the
*Nix mail program (postfix, sendmail, etc.) to allow longer send times, but
obviously I'm running Windows.

I'm at a loss as to where to find more detailed error messages, or what
else to try.

I'm sorry if I missed an obvious answer, somewhere. Any thoughts?

To reiterate: Only errors with attached file size > 1.5MB. Smaller
attachments work very well.

TIA.