[wp-trac] [WordPress Trac] #15448: wp_mail() sets Content-Type header twice for multipart emails

Mon Sep 1 16:55:31 UTC 2025

#15448: wp_mail() sets Content-Type header twice for multipart emails
-------------------------------------------------+-------------------------
 Reporter:  rmccue                               |       Owner:  SirLouen
     Type:  defect (bug)                         |      Status:  accepted
 Priority:  high                                 |   Milestone:  6.9
Component:  Mail                                 |     Version:  2.8
 Severity:  normal                               |  Resolution:
 Keywords:  has-patch has-unit-tests has-test-   |     Focuses:
  info                                           |
-------------------------------------------------+-------------------------

Comment (by sajjad67):

 == Test Report
 === Description
 This report validates whether the indicated patch works as expected.

 Patch tested: https://github.com/WordPress/wordpress-
 develop/pull/9500.patch

 === Environment
 - WordPress: 6.9-alpha-60093-src
 - PHP: 8.2.29
 - Server: nginx/1.29.1
 - Database: mysqli (Server: 8.4.6 / Client: mysqlnd 8.2.29)
 - Browser: Chrome 139.0.0.0
 - OS: Windows 10/11
 - Theme: Twenty Twenty-Five 1.3
 - MU Plugins: None activated
 - Plugins:
   * Amazon S3 Email Testing 1.0.0
   * Manual Multipart/Alternative/Mixed Test 1.0.0
   * Test Reports 1.2.0
   * WP Mail SMTP 4.6.0

 === Actual Results
 1.  ✅ Emails sent via wp_mail() with multipart content are now parsed
 correctly.
 2.  ✅ Mailpit displays plain text and HTML parts in their respective
 tabs.
 3.  ✅ Attachments (e.g., attachment.pdf) are properly recognized and
 accessible.
 4.  ✅ MIME boundaries and headers are no longer visible in the email
 body.

 === Additional Notes
 Before applying the patch, Mailpit displayed the raw multipart structure
 with MIME headers inline, and attachments were not detected.
 After applying the patch, the behavior aligns with the expected PHPMailer
 handling of multipart messages. Also, no regressions observed in basic
 wp_mail() functionality.

 === Supplemental Artifacts
 Raw email after the patch (Multipart/Alternative Email Test) -

 [[Image(https://img001.prntscr.com/file/img001/eF41u4l8T1KvVXP6ym1JkA.png)]]

 {{{
 Return-Path: <sender at test.com>
 Received: from localhost (wordpress-develop-php-1.wordpress-
 develop_wpdevnet. [172.18.0.3])
         by e28520ee0bd5 (Mailpit) with SMTP
         for <test at test.com>; Mon, 1 Sep 2025 16:40:16 +0000 (UTC)
 Date: Mon, 1 Sep 2025 16:40:16 +0000
 To: test at test.com
 From: WordPress Development <sender at test.com>
 Subject: Multipart/Alternative Email Test
 Message-ID: <a9gUEp7IlF521nXSVU5fYOL0JSXLUiYkIdc5zFlJYI at localhost>
 X-Mailer: WPMailSMTP/Mailer/smtp 4.6.0
 MIME-Version: 1.0
 Content-Type: multipart/mixed;
  boundary="b1=_a9gUEp7IlF521nXSVU5fYOL0JSXLUiYkIdc5zFlJYI"
 Content-Transfer-Encoding: 8bit

 --b1=_a9gUEp7IlF521nXSVU5fYOL0JSXLUiYkIdc5zFlJYI
 Content-Type: multipart/alternative; boundary="h2QJgSwi06UTDAMj2bci6QjM";
 charset=us-ascii

 --h2QJgSwi06UTDAMj2bci6QjM
 Content-Type: text/plain; charset=UTF-8
 Content-Transfer-Encoding: 7bit

 This is the plain text version of the email.
 --h2QJgSwi06UTDAMj2bci6QjM
 Content-Type: text/html; charset=UTF-8
 Content-Transfer-Encoding: 7bit

 <html><body><p>This is the <strong>HTML version</strong> of the
 email.</p></body></html>
 --h2QJgSwi06UTDAMj2bci6QjM--

 --b1=_a9gUEp7IlF521nXSVU5fYOL0JSXLUiYkIdc5zFlJYI
 Content-Type: application/octet-stream; name=attachment.pdf
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename=attachment.pdf

 ....
 RkY1RDc4QjhGMjg+IF0KPj4Kc3RhcnR4cmVmCjEyNzg3CiUlRU9GCg==

 --b1=_a9gUEp7IlF521nXSVU5fYOL0JSXLUiYkIdc5zFlJYI--
 }}}

 Raw email after the patch (Multipart/Mixed Email Test) -
 [[Image(https://img001.prntscr.com/file/img001/lt688EYwQAqOFLkm2yv7Dw.png)]]
 [[Image(https://img001.prntscr.com/file/img001/0UvS1y6tRTq81A11cFGW8w.png)]]

 {{{
 Return-Path: <sender at test.com>
 Received: from localhost (wordpress-develop-php-1.wordpress-
 develop_wpdevnet. [172.18.0.3])
         by e28520ee0bd5 (Mailpit) with SMTP
         for <test at test.com>; Mon, 1 Sep 2025 16:41:13 +0000 (UTC)
 Date: Mon, 1 Sep 2025 16:41:13 +0000
 To: test at test.com
 From: WordPress Development <sender at test.com>
 Subject: Multipart/Mixed Email Test
 Message-ID: <aZAPqJglmQhnxi5DU712pYM8Pma5aEpMslG2nl7Y at localhost>
 X-Mailer: WPMailSMTP/Mailer/smtp 4.6.0
 MIME-Version: 1.0
 Content-Type: multipart/mixed;
  boundary="b1=_aZAPqJglmQhnxi5DU712pYM8Pma5aEpMslG2nl7Y"
 Content-Transfer-Encoding: 8bit

 --b1=_aZAPqJglmQhnxi5DU712pYM8Pma5aEpMslG2nl7Y
 Content-Type: multipart/mixed; boundary="8z4V5qzeofKwtWjpe9HO05Vl";
 charset=us-ascii

 --8z4V5qzeofKwtWjpe9HO05Vl
 Content-Type: text/plain; charset=UTF-8
 Content-Transfer-Encoding: 7bit

 This is a multipart/mixed email with an attachment.

 --8z4V5qzeofKwtWjpe9HO05Vl
 Content-Type: text/plain; name="test-attachment.txt"
 Content-Disposition: attachment; filename="test-attachment.txt"
 Content-Transfer-Encoding: base64

 VGhpcyBpcyB0aGUgY29udGVudCBvZiB0aGUgdGVzdCBhdHRhY2htZW50Lg==

 --8z4V5qzeofKwtWjpe9HO05Vl--

 --b1=_aZAPqJglmQhnxi5DU712pYM8Pma5aEpMslG2nl7Y
 Content-Type: application/octet-stream; name=attachment.pdf
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename=attachment.pdf

 ....
 RkY1RDc4QjhGMjg+IF0KPj4Kc3RhcnR4cmVmCjEyNzg3CiUlRU9GCg==

 --b1=_aZAPqJglmQhnxi5DU712pYM8Pma5aEpMslG2nl7Y--
 }}}

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/15448#comment:92>
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