By default, the list of bookings with unread emails will be sorted by the newest messages first, similarly to how a normal inbox would appear. You also have the option by ordering them as to how you want by using the sorting tool at the top of the list.
Here you can sort the order by
Relevance: This will order the list with the later bookings first unless searching, in which case the best match should be first
Date (earliest first): This will order the booking list with the date that's earliest at the top(and include past dates if there are incoming messages from old bookings) and descend in order underneath.
Date (latest first): The list will be ordered by the booking date that's furthest away from today's date at the top