Cart66 not sending emails to customers

If email receipts are not being sent/received, it’s almost always because your web server’s open relay mail system is getting blocked by spam filters. The best thing to do is to create a real SMTP account either alongside your regular email accounts or with a service like Mandrill http://mandrill.com (free for up to 12,000 messages) then use a plugin like WP Mail SMTP http://wordpress.org/extend/plugins/wp-mail-smtp/