Allow creating custom "Universal inboxes" or multi filter dynamic folders
Mac Mail allows us to create dynamic filtered folders from several criteria.
In my case I have multiple accounts and within those accounts I have some subfolders.
For general browsing I would like to be able to choose whichever multiple folder to display in one single dynamic inbox.
Either that or allowing the creation of dynamic universal inboxes, set by each message criteira.

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coffee_coffee_coffee commented
Yes! Custom inboxes such as the Boxer iOS app. I want to have a universal inbox and also have an inbox that only threads the messages of a subset of selected accounts. As far as I can tell I can edit the one universal inbox with Airmail but not create additional ones.