Content-Type: multipart/alternative in WordPress with wp_mail()

3 comments

1. It’s right there in the wp_mail() documentation (under Usage):

   The default content type is ‘text/plain’ which does not allow using HTML. You can set the content type of the email either by using the ‘wp_mail_content_type’ filter (see example below), or by including a header like “Content-type: text/html”. Be careful to reset ‘wp_mail_content_type’ back to ‘text/plain’ after you send your message, though, because failing to do so could lead to unexpected problems with e-mails from WP or plugins/themes.

   (emphasis mine)

   The 2nd example on the page shows you how to do it (the example uses text/html but you should be able to use your multipart/alternative instead.

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2. It’s totally possible when you have access to the phpmailer instance.

   if ($is_html) 
       add_action('phpmailer_init', 'fix_mimeheader');
   
   // more code.
   
   wp_mail( $to, $subject, $html_message, $headers, $attachments );
   
   // ...
   
   function fix_mimeheader( $phpmailer ) {
        // Generate $text_message here.
        // ...
   
        $phpmailer->AltBody = $text_message;
   }
   

   The message sent to wp_mail should be your html code. You also shouldn’t include any content type headers. I currently use from, cc and reply-to in the plugin i’ve made.

   If the email is being sent as HTML, I run the action which sets the AltBody property on the phpmailer object directly. This then causes the proper flags to convert the email to a multipart/alternative email.

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3. You can use the wp_mail_content_type filter, which was now been documented in the Codex.

   The wp_mail documentation about resetting the content type back to ‘text/plain’ is kind of misleading, IMO. Since this is a filter, you don’t really “reset” it. What you need to consider in your filter is some conditional logic to determine when you need to use multipart vs. plain text or html:

   add_filter( 'wp_mail_content_type', 'my_mail_content_type' );
   function my_mail_content_type( $content_type ) {
   
       if( $some_condition ) {
           return 'multipart/mixed';
       } else {
           return 'text/plain';
       }
   }
   

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