Airmail 3 has now a new feature called ‘Smart Folders’. You can now save your searches as ‘Smart Folders’.
These ‘virtual’ folders just save your search with a particular criterion so that, you don’t have to search for the same criterion again and again. ‘Virtual’ in this case means that they don’t have the messages’ copies in them for real. They just fetch the search results in one place.
Smart folders can be made to appear in Sidebar just as the iOS Airmail app, for easy access. Please see the screenshots.
How to create a Smart Folder?
You can access the option to create a new smart folder in the search bar itself. Please refer to this picture
How to set the search criteria for a smart folder?
You can use the pre-defined operators which are already indicated in the smart folder box as shown in the picture below. These operators should be sufficient for almost all the quick searches. If you still need to define extra operators for the smart folder, you can use the Google Advanced Search Operators https://support.google.com/mail/answer/7190?hl=en
How to include the Smart Folders in the sidebar?
You have to just click on the three dash icon and select the Smart Folder option to enable them to appear in the sidebar.