Microsoft Exchange 不会将 PHPmailer 生成的电子邮件呈现为 HTML

这个问题已经困扰我好几个星期了。我有一个脚本，可以在 PHPmailer 的帮助下将带有 xls 附件的 html 电子邮件发送给多个收件人。它已经运行良好一年多了。最近，来自同一家公司的两个使用 Microsoft Exchange 作为电子邮件客户端的收件人收到了这封文本电子邮件。以下是他们收到的电子邮件示例：

--b1_dbc2cccc876da210fd56ae7a2601e692
Content-Type: multipart/alternative;
        boundary="b2_dbc2cccc876da210fd56ae7a2601e692"

--b2_dbc2cccc876da210fd56ae7a2601e692
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

To view the message, please use an HTML compatible email viewer!


--b2_dbc2cccc876da210fd56ae7a2601e692
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 8bit

....................................
the email body html code in this block
....................................


--b2_dbc2cccc876da210fd56ae7a2601e692--

--b1_dbc2cccc876da210fd56ae7a2601e692
Content-Type: application/octet-stream; name="3MShop-NewOrders_07-30AM-05052014.xls"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="3MShop-NewOrders_07-30AM-05052014.xls"

PGgyPjNNU2hvcC1OZXdPcmRlcnMgMDUtMDUtMjAxNCAwNzozMDoxMCBBTSA8L2gyPjxociAvPjx0
YWJsZSBib3JkZXI9MSBjZWxscGFkZGluZz01IGNlbGxzcGFjaW5nPTI+PHRyPjx0ZCBhbGln

MDUgLSBOYXZ5IEJsdWU8L3RkPjx0ZCBhbGlnbj1sZWZ0PjE8L3RkPjwvdHI+PC90YWJsZT4=

....................................
the rest codes of this attachment
....................................

--b1_dbc2cccc876da210fd56ae7a2601e692--

以下是其他收件人收到的电子邮件标头示例

Content-Type: multipart/mixed;
    boundary="b1_dbc2cccc876da210fd56ae7a2601e692"
Sender:  <[email protected] /cdn-cgi/l/email-protection>
X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - cp60.ezyreg.com
X-AntiAbuse: Original Domain - emoceanstudios.com.au
X-AntiAbuse: Originator/Caller UID/GID - [616 500] / [47 12]
X-AntiAbuse: Sender Address Domain - emoceanstudios.com.au
X-Get-Message-Sender-Via: cp60.ezyreg.com: authenticated_id: emocean/from_h
X-Source: 
X-Source-Args: /usr/sbin/proxyexec -q -d -s /var/run/proxyexec/cagefs.sock/socket /bin/cagefs.server 
X-Source-Dir: emoceanstudios.com.au:/public_html/3M-Shop-Scripts

--b1_dbc2cccc876da210fd56ae7a2601e692
Content-Type: multipart/alternative;
    boundary="b2_dbc2cccc876da210fd56ae7a2601e692"

--b2_dbc2cccc876da210fd56ae7a2601e692
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

To view the message, please use an HTML compatible email viewer!


--b2_dbc2cccc876da210fd56ae7a2601e692
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 8bit

正如您可以轻松看到的区别是有问题的部分没有

Content-Type: multipart/mixed;

更奇怪的是，虽然他们的 Microsoft Exchange 无法将从 PHPmailer 发送的这封电子邮件呈现为 HTML，但如果我将同一封电子邮件从我的 gmail 转发到他们的 Exchange，他们现在可以正确呈现它。这让我觉得这可能是我的 php 脚本问题。脚本在这里：

$mail = new PHPMailer(true);
$mail->CharSet = 'UTF-8';


$mail->AddAddress('[email protected] /cdn-cgi/l/email-protection', 'example name');
$mail->SetFrom('[email protected] /cdn-cgi/l/email-protection', 'example name');

$mail->ContentType = 'text/html';
$mail->IsHTML(true);  

$mail->Subject = 'example subject';
$mail->AltBody = 'To view the message, please use an HTML compatible email viewer!'; 

$mail->MsgHTML(file_get_contents('contents.html'));
$mail->AddAttachment($newOrdersReport);      // attachment

$mail->Send();

我不是电子邮件专家，但这对我来说看起来不合适：

--b1_dbc2cccc876da210fd56ae7a2601e692
Content-Type: multipart/alternative;
boundary="b2_dbc2cccc876da210fd56ae7a2601e692"

您已经将 Content-Type 声明为 multipart 并定义了边界，为什么会重复？