[wp-trac] [WordPress Trac] #43542: Duplicate MIME-Version header

[WordPress Trac]

#43542: Duplicate MIME-Version header
--------------------------+-----------------------------
 Reporter:  junktrunk     |      Owner:
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Mail          |    Version:  4.9.4
 Severity:  normal        |   Keywords:
  Focuses:                |
--------------------------+-----------------------------
 Issue: Duplicate "MIME-Version" header information can be sent via
 pluggable.php and class-phpmailer.php via plugins, causing the mail to
 fail to be delivered when using a service like AWS SMTP that rejects this
 duplicate header element.

 I see that this issue has been reported in the past but closed as an
 invalid bug (e.g. https://core.trac.wordpress.org/ticket/22492 from 5
 years ago).  But I argue that that may not be the correct conclusion.

 Wordpress plugin developer guidance on the use of wp_mail() doesn't
 prohibit including MIME-Version in  $headers , and does otherwise offer to
 "build the proper Mime header automagically".  I have read an argument
 online that it may be proper for the plugin to build out the full header
 including the mandatory MIME-Version in the event the administrator has
 configured their site to use a different email service besides WP/PHP,
 though I haven't substantiated that fully.

 Personally I think this should be a configurable option within any plugin,
 defaulted to off, if included at all.  But in googling this issue, I see
 that this problem occurs in a large number of plugins, and for years,
 going unfixed, resulting in probably millions of unsent messages. For
 example, the following bug report for Email Subscription (the plugin I am
 working with that caused me to re-re-discover this problem) is from 2014,
 where it's still a problem today: https://github.com/Nyholm/Wordpress-
 Email-Subscription/issues/10

 Regardless of the best component where this element should be added, the
 default installation of WP should have checks to avoid duplication that
 causes failures, especially given there is an easy fix.  In pluggable.php,
 in the loop processing each of the $headers the plugin supplied, we can
 check to see whether any of those will be duplicated by class-
 phpmailer.php, omitting them if so:

 {{{
         // Set custom headers
         if ( !empty( $headers ) ) {
                 foreach ( (array) $headers as $name => $content ) {
                         // Only add custom header if not one that will be
 added automatically
                         // by wp-includes/class-phpmailer.php
                         if (('MIME-Version' !== $name) && ('X-Mailer' !==
 $name))
                                 $phpmailer->addCustomHeader( sprintf(
 '%1$s: %2$s', $name, $content ) );
                 }

 }}}

 (In my code snippet above I included X-Mailer too because that's also
 added automatically by class-phpmailer.php and doesn't make sense to
 duplicate, though it is not causing the same mail delivery failure when
 duplicated.)

--
Ticket URL: <https://core.trac.wordpress.org/ticket/43542>
WordPress Trac <https://core.trac.wordpress.org/>
WordPress publishing platform