add_filter( 'retrieve_password_message', 'my_retrieve_password_message', 10, 4 );
function my_retrieve_password_message( $message, $key, $user_login, $user_data ) {

    // Start with the default content.
    $site_name = wp_specialchars_decode( get_option( 'blogname' ), ENT_QUOTES );
    $message = __( 'Someone has requested a password reset for the following account:' ) . "\r\n\r\n";
    /* translators: %s: site name */
    $message .= sprintf( __( 'Site Name: %s' ), $site_name ) . "\r\n\r\n";
    /* translators: %s: user login */
    $message .= sprintf( __( 'Username: %s' ), $user_login ) . "\r\n\r\n";
    $message .= __( 'If this was a mistake, just ignore this email and nothing will happen.' ) . "\r\n\r\n";
    $message .= __( 'To reset your password, visit the following address:' ) . "\r\n\r\n";
    $message .= '<' . network_site_url( "wp-login.php?action=rp&key=$key&login=" . rawurlencode( $user_login ), 'login' ) . ">\r\n";

    /*
     * If the problem persists with this filter, remove
     * the last line above and use the line below by
     * removing "//" (which comments it out) and hard
     * coding the domain to your site, thus avoiding
     * the network_site_url() function.
     */
    // $message .= '<http://yoursite.com/wp-login.php?action=rp&key=' . $key . '&login=' . rawurlencode( $user_login ) . ">\r\n";

    // Return the filtered message.
    return $message;

}

add_filter( 'retrieve_password_title', 
    function( $title ) 
    {
        $title = __( 'WPSE Custom Password Reset' );
        return $title;
    }
);