Prevent email addresses from being added to Pardot if they end with certain domain names.

gistfile1.php

<?php
add_filter( 'optin_monster_pre_optin_pardot', 'tgm_om_block_specific_email_domains' );
function tgm_om_block_specific_domains( $data ) {
	
	// If no email passed, return.
	if ( empty( $data['email'] ) ) {
		return $data;
	}
	
	// Check for specific email domains. If there is a match, set email to false to prevent from sending to Pardot.
	if ( strpos( $data['email'], '@gmail.com' ) !== false || strpos( $data['email'], '@hotmail.com' ) !== false || strpos( $data['email'], '@yahoo.com' ) !== false ) {
		$data['email'] = false;
	}
	
	// Return the potentially modified data.
	return $data;
	
}