In a mass mailing, users can set an event related and exclude the recipients who
have an email address already registered in that event.

This is useful in this scenario:

1. Create a mass mailing for telling to 1000 partners that a new event is
available.
2. During a week some of them (50) have been registered in the event
3. Then, a week after the first mass mailing, duplicate it to get a second
mass mailing. Relate this one to the event and exclude the registered emails.
Change the message body to remember that early bird period is going to expire
soon.
4. Send the second mass mailing and registered emails are automatically excluded,
So it's been only sent to 950 partners the ones who are not registered in the
event yet.
