An email with no subject is highly suspect - a lot of people will treat it as spam.

We can't just make the EmailSubject field itself required, because the CMS interface allows you to set a field from the form itself to be the source of the subject.

I recommend the following solution:

• One of EmailSubject or SendEmailSubjectField must have a value.
• If SendEmailSubjectField is used, the field it points to must be a required field.
• If, when we finally go to send the email, the subject is somehow empty, a warning is logged.