Filter "wp_mail" - How to distinguish between a WordPress system email and a plugin email?

Question

Using the wp_mail filter I add a "Reply-To" to the header with my email address, so clients can reply directly to the email to contact me if there's any issues. I only want this on WordPress "system" emails, like emails for new accounts, password resets, etc. If a plugin sends an email using wp_mail() I don't want to set the Reply To address. Is there some way to know when it's WordPress core sending an email rather than a plugin?

Here's what I'm using to set the Reply-To (from a custom class):

public function __construct() {
    add_filter( 'wp_mail', array( $this, 'setReplyTo' ), PHP_INT_MAX );
}

public function setReplyTo( array $args ) : array
{
    $reply_to = 'Reply-To: My Name <[email protected]>';
    
    if ( ! empty( $args['headers'] ) ) {
        if ( ! is_array( $args['headers'] ) ) {
            $args['headers'] = array_filter( explode( "\n", str_replace( "\r\n", "\n", $args['headers'] ) ) );
        }
        // Remove existing Reply-To headers
        $args['headers'] = array_filter( $args['headers'], function( $header ) {
            return strpos( strtolower( $header ), 'reply-to' ) !== 0;
        } );
    }
    else {
        $args['headers'] = [];
    }

    $args['headers'][] = $reply_to;

    return $args;
}

Nothing in the $args parameter appears to contains anything I can use to accomplish this. Hopefully there's some way to know what is initiating the emails. Thanks!

Answer 1

I agree with @birgire that #53829-core would be the ideal way to do this. If you need a hack that works in the meantime, though, then you can examine the filenames in the call stack.

add_filter( 'wp_mail', 'modify_system_emails' );

function modify_system_emails( array $attributes ) : array {
    $wp_mail_caller = get_wp_mail_caller();

    if ( is_core_file( $wp_mail_caller['file'] ) ) {
        $attributes['subject'] .= ' -- (system)';
    }

    return $attributes;
}

function get_wp_mail_caller() : array {
    $backtrace = debug_backtrace( DEBUG_BACKTRACE_IGNORE_ARGS );

    foreach ( $backtrace as $call ) {
        if ( 'wp_mail' === $call['function'] ) {
            return $call;
        }
    }

    throw new Exception( "`wp_mail()` wasn't found in the backtrace." );
}

function is_core_file( string $filename ) : bool {
    $relative_path = str_replace( ABSPATH, '', $filename );
    $parts         = explode( '/', $relative_path );

    if ( in_array( $parts[0], [ 'wp-admin', 'wp-includes' ], true ) ) {
        return true;
    } elseif ( preg_match( '#^wp-.*\.php$#', $parts[0] ) ) {
        return true;
    }

    return false;
}

You can test that with some code like:

add_action( 'init', function() {
    wp_mail( '[email protected]', 'not a system email', 'nosirree' ); // This won't modify the subject.
    wp_site_admin_email_change_notification( '[email protected]', '[email protected]', 'foo' ); // This will modify the subject.
} );