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

Question

Is it is possible to send emails with the wp_mail() function having its Content-Type: multipart/alternative ?

I need to send emails that can be shown as HTML or Plain Text depending on what medium interprets the email.

Any suggestions are welcome!

Answer 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.

Answer 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.

Answer 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';
    }
}