It is now possible to cancel a newsletter after it has started sending or has started queuing (calculating segments).

When cancelling a queuing newsletter, it will not send to any customers and return to a draft state so you can schedule it at a later date.

When cancelling a sending newsletter it will enter a 'cancelled' state, however depending on the size of the campaign some customers may have already received the campaign, for this reason it cannot be rescheduled and will need to be cloned with a 'has not received campaign' condition to ensure customers who already received the campaign don't receive a duplicate.