mepr_validate_emails_signup.php

<?php

function mepr_validate_emails_signup($errors) {
  $allowed_words = array('university'); // ADD ALLOWED WORDS HERE SEPARATED BY COMMAS.
  $email = isset($_POST['user_email']) ? trim($_POST['user_email']) : '';
  $is_allowed = false;

  // If email is empty for some reason or the membership shouldn't be validated, then just don't do anything.
  if($_POST['mepr_product_id'] != 123 || empty($email)) {
    return $errors;
  }

  // Loop over allowed words.
  foreach($allowed_words as $allowed_word) {
    if(strpos($email, $allowed_word) !== false) {
      $is_allowed = true;
    }
  }

  if(!$is_allowed) {
    $errors[] = 'Your email isn\'t allowed to sign up.';
  }

  return $errors;

}
add_filter('mepr-validate-signup', 'mepr_validate_emails_signup');