I think you need to change it to this:
if (!eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$", $customer_eMail))
Notice the last part, I think this enlarges it to 4 characters. As you have it, it rejects any 4 character extension.