How validate an email address in PHP (by domain)

class-validate-email.php

<?php
/**
 * Validates email addresses
 *
 * This class is used to validate email addresses using several different criteria.
 *
 * @version 1.0
 * @author John Morris <support@johnmorrisonline.com>
 */
class validate_email {
  /**
   * Accepted domains
   * @access private
   * @var array
   */
  private $accepted_domains;
  
  /**
   * The class constructor
   * 
   * This sets up the class
   */
  public function __construct() {
    // Set the accepted domains property
    $this->accepted_domains = array(
      'unl.edu', 'uni.edu'
    );
  }
  
  /**
   * Validate email address by domain
   * 
   * Checks if the email address belongs to an accepted domain
   */
  public function validate_by_domain($email_address) {
    // Get the domain from the email address
    $domain = $this->get_domain( trim( $email_address ) );
    
    // Check if domain is accepted. Return return if so
    if ( in_array( $domain, $this->accepted_domains ) ) {
      return true;
    }
    
    return false;
  }
  
    /**
   * Get the domain
   * 
   * Gets the domain from an email address
   */
  private function get_domain($email_address) {
    // Check if a valid email address was submitted
    if ( ! $this->is_email( $email_address ) ) {
      return false;
    }
    
    // Split the email address at the @ symbol
    $email_parts = explode( '@', $email_address );
    
    // Pop off everything after the @ symbol
    $domain = array_pop( $email_parts );
    
    return $domain;
  }
  
  /**
   * Check email address
   * 
   * Checks if the submitted value is a valid email address
   */
  private function is_email($email_address) {
    // Filter submitted value to see if it's a proper email address
    if ( filter_var ( $email_address, FILTER_VALIDATE_EMAIL ) ) {
      return true;
    }
    
    return false;
  }
}

index.php

<?php
require_once( dirname( __FILE__ ) . '/class-validate-email.php' );

$validate_email = new validate_email;

// Valid
if ( $validate_email->validate_by_domain('justin@unl.edu ') ) {
  echo '<p>This is valid!</p>';
} else {
  echo '<p>This is NOT valid!</p>';
}

// Valid
if ( $validate_email->validate_by_domain('jason@uni.edu') ) {
  echo '<p>This is valid!</p>';
} else {
  echo '<p>This is NOT valid!</p>';
}

// Not valid
if ( $validate_email->validate_by_domain('ramone@ui.edu') ) {
  echo '<p>This is valid!</p>';
} else {
  echo '<p>This is NOT valid!</p>';
}