Enable "Default On" field

If we have a customer for whom no email address is known, we—understandably—cannot send an email. However, if I’m working on a ticket and need to email a third party, I can only do so by adding the email address to the customer’s record. In the case of entities with multiple customers (such as DNS hosts), this isn’t particularly practical. Is it possible to have the email address field set to “on” by default, even if the contact person’s email address isn’t known?