Utility class with a helper method that lets me check if the incoming email address input by the user matches a given domain I am verifying against. See the corresponding test: https://gist.github.com/kevinquillen/85baa1fa69f49d1c34aa81369405d47e

EmailMatcher.php

<?php

namespace Drupal\iana_netforum_auth\Utility;

/**
 * Class EmailMatcher.
 *
 * @package Drupal\iana_netforum_auth\Utility
 */
class EmailMatcher {

  /**
   * Check if the email address entered matches the domain.
   *
   * @param string $email_address
   *    The provided email address.
   * @param string $domain
   *    The domain to match it against.
   * @return bool
   *    TRUE if the email domain and provided domain match.
   */
  public static function emailDomainMatch(string $email_address, string $domain) : bool {
    $check_for = explode('@', $email_address);

    // Invalid email address.
    if (count($check_for) != 2) {
      return FALSE;
    }

    return (end($check_for) === $domain);
  }

}