validate_email() function with transients

plugin.php

<?php
                //Function which sends the email to Mailgun to check it
                public function validate_email( $emailID ) {
                        global $pagenow, $wp;
                        //If the format of the email itself is wrong return false without further checking
                        if( ! filter_var( $emailID, FILTER_VALIDATE_EMAIL ) )
                                return FALSE;

                        //If no API was entered don't do anything
                        if( ! isset( $this->options['mailgun_pubkey_api'] ) || empty( $this->options['mailgun_pubkey_api'] ) )
                                return TRUE;

                        if ( "edit.php" == $pagenow && "shop_order" == $wp->query_vars['post_type'] ) {
                                return true;
                        }

                        $emails = get_transient( 'mailgun_validated_emails' );
                        $emailID = strtolower( $emailID );

                        if ( isset( $emails[ $emailID ] ) ) {
                                return ( 1 == $emails[ $emailID ] ) ? $emailID : false;
                        }

                        $args = array(
                                'sslverify' => FALSE,
                                'headers' => array(
                                        'Authorization' => 'Basic ' . base64_encode( "api:".$this->options['mailgun_pubkey_api'] )
                                )
                        );
                        //Send the email to Mailgun's email validation service
                        $response = wp_remote_request( "https://api.mailgun.net/v3/address/validate?address=".urlencode($emailID), $args );

                        //If there was a HTTP or connection error pass the validation so that the website visitor doesn't know anything
                        if( is_wp_error( $response ) || isset( $response['error'] ) || '200' != $response['response']['code'] )
                                return TRUE;

                        //Extract the JSON response and return the result
                        $result = json_decode( $response['body'], true );

                        if ( ! $emails ) $emails = array();
                        $emails[ $emailID ] = ( $result['is_valid'] ? '1' : '0' );
                        set_transient( 'mailgun_validated_emails', $emails, MONTH_IN_SECONDS );

                        return $result['is_valid'] ? $emailID : false;
                 }