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I am passing a custom header multiple times, for tagging reasons, to wp_mail(), but only the last instance of the header is actually getting sent. It appears that wp_mail() is filtering the custom headers so only unique ones are sent.

I believe this is occurring because in the wp_mail function definition (wp-includes/pluggable.php, line 173) custom headers are processed by being added to an associative array (line 303):

   $headers[trim( $name )] = trim( $content );

Is there a way to alter this functionality without modifying the wp_mail function itself?

WP Codex for wp_mail()

wp_mail() Source code

function wp_mail( $to, $subject, $message, $headers = '', $attachments = array() ) {
    // Compact the input, apply the filters, and extract them back out

    /**
     * Filters the wp_mail() arguments.
     *
     * @since 2.2.0
     *
     * @param array $args A compacted array of wp_mail() arguments, including the "to" email,
     *                    subject, message, headers, and attachments values.
     */
    $atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );

    if ( isset( $atts['to'] ) ) {
        $to = $atts['to'];
    }

    if ( !is_array( $to ) ) {
        $to = explode( ',', $to );
    }

    if ( isset( $atts['subject'] ) ) {
        $subject = $atts['subject'];
    }

    if ( isset( $atts['message'] ) ) {
        $message = $atts['message'];
    }

    if ( isset( $atts['headers'] ) ) {
        $headers = $atts['headers'];
    }

    if ( isset( $atts['attachments'] ) ) {
        $attachments = $atts['attachments'];
    }

    if ( ! is_array( $attachments ) ) {
        $attachments = explode( "\n", str_replace( "\r\n", "\n", $attachments ) );
    }
    global $phpmailer;

    // (Re)create it, if it's gone missing
    if ( ! ( $phpmailer instanceof PHPMailer ) ) {
        require_once ABSPATH . WPINC . '/class-phpmailer.php';
        require_once ABSPATH . WPINC . '/class-smtp.php';
        $phpmailer = new PHPMailer( true );
    }

    // Headers
    $cc = $bcc = $reply_to = array();

    if ( empty( $headers ) ) {
        $headers = array();
    } else {
        if ( !is_array( $headers ) ) {
            // Explode the headers out, so this function can take both
            // string headers and an array of headers.
            $tempheaders = explode( "\n", str_replace( "\r\n", "\n", $headers ) );
        } else {
            $tempheaders = $headers;
        }
        $headers = array();

        // If it's actually got contents
        if ( !empty( $tempheaders ) ) {
            // Iterate through the raw headers
            foreach ( (array) $tempheaders as $header ) {
                if ( strpos($header, ':') === false ) {
                    if ( false !== stripos( $header, 'boundary=' ) ) {
                        $parts = preg_split('/boundary=/i', trim( $header ) );
                        $boundary = trim( str_replace( array( "'", '"' ), '', $parts[1] ) );
                    }
                    continue;
                }
                // Explode them out
                list( $name, $content ) = explode( ':', trim( $header ), 2 );

                // Cleanup crew
                $name    = trim( $name    );
                $content = trim( $content );

                switch ( strtolower( $name ) ) {
                    // Mainly for legacy -- process a From: header if it's there
                    case 'from':
                        $bracket_pos = strpos( $content, '<' );
                        if ( $bracket_pos !== false ) {
                            // Text before the bracketed email is the "From" name.
                            if ( $bracket_pos > 0 ) {
                                $from_name = substr( $content, 0, $bracket_pos - 1 );
                                $from_name = str_replace( '"', '', $from_name );
                                $from_name = trim( $from_name );
                            }

                            $from_email = substr( $content, $bracket_pos + 1 );
                            $from_email = str_replace( '>', '', $from_email );
                            $from_email = trim( $from_email );

                        // Avoid setting an empty $from_email.
                        } elseif ( '' !== trim( $content ) ) {
                            $from_email = trim( $content );
                        }
                        break;
                    case 'content-type':
                        if ( strpos( $content, ';' ) !== false ) {
                            list( $type, $charset_content ) = explode( ';', $content );
                            $content_type = trim( $type );
                            if ( false !== stripos( $charset_content, 'charset=' ) ) {
                                $charset = trim( str_replace( array( 'charset=', '"' ), '', $charset_content ) );
                            } elseif ( false !== stripos( $charset_content, 'boundary=' ) ) {
                                $boundary = trim( str_replace( array( 'BOUNDARY=', 'boundary=', '"' ), '', $charset_content ) );
                                $charset = '';
                            }

                        // Avoid setting an empty $content_type.
                        } elseif ( '' !== trim( $content ) ) {
                            $content_type = trim( $content );
                        }
                        break;
                    case 'cc':
                        $cc = array_merge( (array) $cc, explode( ',', $content ) );
                        break;
                    case 'bcc':
                        $bcc = array_merge( (array) $bcc, explode( ',', $content ) );
                        break;
                    case 'reply-to':
                        $reply_to = array_merge( (array) $reply_to, explode( ',', $content ) );
                        break;
                    default:
                        // Add it to our grand headers array
                        $headers[trim( $name )] = trim( $content );
                        break;
                }
            }
        }
    }

    // Empty out the values that may be set
    $phpmailer->clearAllRecipients();
    $phpmailer->clearAttachments();
    $phpmailer->clearCustomHeaders();
    $phpmailer->clearReplyTos();

    // From email and name
    // If we don't have a name from the input headers
    if ( !isset( $from_name ) )
        $from_name = 'WordPress';

    /* If we don't have an email from the input headers default to wordpress@$sitename
     * Some hosts will block outgoing mail from this address if it doesn't exist but
     * there's no easy alternative. Defaulting to admin_email might appear to be another
     * option but some hosts may refuse to relay mail from an unknown domain. See
     * https://core.trac.wordpress.org/ticket/5007.
     */

    if ( !isset( $from_email ) ) {
        // Get the site domain and get rid of www.
        $sitename = strtolower( $_SERVER['SERVER_NAME'] );
        if ( substr( $sitename, 0, 4 ) == 'www.' ) {
            $sitename = substr( $sitename, 4 );
        }

        $from_email = 'wordpress@' . $sitename;
    }

    /**
     * Filters the email address to send from.
     *
     * @since 2.2.0
     *
     * @param string $from_email Email address to send from.
     */
    $from_email = apply_filters( 'wp_mail_from', $from_email );

    /**
     * Filters the name to associate with the "from" email address.
     *
     * @since 2.3.0
     *
     * @param string $from_name Name associated with the "from" email address.
     */
    $from_name = apply_filters( 'wp_mail_from_name', $from_name );

    try {
        $phpmailer->setFrom( $from_email, $from_name, false );
    } catch ( phpmailerException $e ) {
        $mail_error_data = compact( 'to', 'subject', 'message', 'headers', 'attachments' );
        $mail_error_data['phpmailer_exception_code'] = $e->getCode();

        /** This filter is documented in wp-includes/pluggable.php */
        do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

        return false;
    }

    // Set mail's subject and body
    $phpmailer->Subject = $subject;
    $phpmailer->Body    = $message;

    // Set destination addresses, using appropriate methods for handling addresses
    $address_headers = compact( 'to', 'cc', 'bcc', 'reply_to' );

    foreach ( $address_headers as $address_header => $addresses ) {
        if ( empty( $addresses ) ) {
            continue;
        }

        foreach ( (array) $addresses as $address ) {
            try {
                // Break $recipient into name and address parts if in the format "Foo <bar@baz.com>"
                $recipient_name = '';

                if ( preg_match( '/(.*)<(.+)>/', $address, $matches ) ) {
                    if ( count( $matches ) == 3 ) {
                        $recipient_name = $matches[1];
                        $address        = $matches[2];
                    }
                }

                switch ( $address_header ) {
                    case 'to':
                        $phpmailer->addAddress( $address, $recipient_name );
                        break;
                    case 'cc':
                        $phpmailer->addCc( $address, $recipient_name );
                        break;
                    case 'bcc':
                        $phpmailer->addBcc( $address, $recipient_name );
                        break;
                    case 'reply_to':
                        $phpmailer->addReplyTo( $address, $recipient_name );
                        break;
                }
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

    // Set to use PHP's mail()
    $phpmailer->isMail();

    // Set Content-Type and charset
    // If we don't have a content-type from the input headers
    if ( !isset( $content_type ) )
        $content_type = 'text/plain';

    /**
     * Filters the wp_mail() content type.
     *
     * @since 2.3.0
     *
     * @param string $content_type Default wp_mail() content type.
     */
    $content_type = apply_filters( 'wp_mail_content_type', $content_type );

    $phpmailer->ContentType = $content_type;

    // Set whether it's plaintext, depending on $content_type
    if ( 'text/html' == $content_type )
        $phpmailer->isHTML( true );

    // If we don't have a charset from the input headers
    if ( !isset( $charset ) )
        $charset = get_bloginfo( 'charset' );

    // Set the content-type and charset

    /**
     * Filters the default wp_mail() charset.
     *
     * @since 2.3.0
     *
     * @param string $charset Default email charset.
     */
    $phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );

    // Set custom headers
    if ( !empty( $headers ) ) {
        foreach ( (array) $headers as $name => $content ) {
            $phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
        }

        if ( false !== stripos( $content_type, 'multipart' ) && ! empty($boundary) )
            $phpmailer->addCustomHeader( sprintf( "Content-Type: %s;\n\t boundary=\"%s\"", $content_type, $boundary ) );
    }

    if ( !empty( $attachments ) ) {
        foreach ( $attachments as $attachment ) {
            try {
                $phpmailer->addAttachment($attachment);
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

    /**
     * Fires after PHPMailer is initialized.
     *
     * @since 2.2.0
     *
     * @param PHPMailer $phpmailer The PHPMailer instance (passed by reference).
     */
    do_action_ref_array( 'phpmailer_init', array( &$phpmailer ) );

    // Send!
    try {
        return $phpmailer->send();
    } catch ( phpmailerException $e ) {

        $mail_error_data = compact( 'to', 'subject', 'message', 'headers', 'attachments' );
        $mail_error_data['phpmailer_exception_code'] = $e->getCode();

        /**
         * Fires after a phpmailerException is caught.
         *
         * @since 4.4.0
         *
         * @param WP_Error $error A WP_Error object with the phpmailerException message, and an array
         *                        containing the mail recipient, subject, message, headers, and attachments.
         */
        do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

        return false;
    }
}
  • Please, show us the code used to create headers. – Max Yudin Feb 10 '18 at 13:04
  • you can use the action phpmailer_init to custom the instance of PHPMailer before the e-mail is send. – mmm Feb 10 '18 at 13:08
  • @MaxYudin The headers are a simple array of strings: `$headers = [ 'X-Custom-Header: Val1', 'X-Custom-Header: Val2', ... ] – bicarlsen Feb 10 '18 at 14:04
  • @mmm I don't believe modifying the phpmailer_init hook will solve this problem. could you elaborate on what you were thinking? – bicarlsen Feb 10 '18 at 14:07
  • @bicarlsen, why don't you add this essential information into your question? Conversation is OK but read previous comments also. Code! We need the code! Otherwise minus. – Max Yudin Feb 10 '18 at 23:09
-1

You use an index in your $header variable. AFAIK this should be left empty.

Try $headers[] = trim( $content );

| improve this answer | |
  • Sorry for the confusions, the line I posted $headers[trim( $name )] = trim( $content ); is from the wp_mail function definition (line 303) – bicarlsen Feb 10 '18 at 14:05
  • I stand corrected :) – Beee Feb 11 '18 at 18:47

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