In Airmail you can use the oldest message on top and the newest message at the bottom. If you are reading mails from the oldest then we may suggest you to enable the option that allows you to reverse the order your mails and you can also archive them if you don’t wish to keep them in the Inbox.
The option to reverse sort order is located under Window in the Menu Title/Bar.
On the following screenshot below there is an option for sorting mails in reverse order:
Choosing this option may also rewrite the new database for Airmail but will not affect Web Mail.