Convert Drupal 7 Email field to drupal 8 Processor

EmailConvert.php

<?php

namespace Drupal\base\Plugin\migrate\process;

use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;

/**
 * This plugin converts Drupal 7 email field to drupal 8
 *
 * field_mail:
 *   plugin: email_convert
 *   source: field_e_mail
 *  
 * @MigrateProcessPlugin(
 *   id = "email_convert"
 * )
 */
class EmailConvert extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (is_array($value) && isset($value['email'])) {
      return $value['email'];
    }
    return $value;
  }

}

Alternatively you can just add..

  field_new_field:
    plugin: extract
    source: field_old_field
    index:
      - email