[AdieuToLogic]

> Are there mail clients that actually support things like priority inbox and categorization that don't simply crumble for large inboxes?

Of the exemplars listed, nmh is by far the most flexible and capable of handling large inboxes. The reason why is that it is a set of command-line executables which can be scripted with whatever language you prefer. So prioritization and/or categorization is a matter of how one wants each to be defined.

I used IMAP via fetchmail[0] with nmh to allow multiple MUAs (on different machines and/or OSs) access/management to the same email accounts with great success. IIRC, for nmh I used lynx to render HTML messages and xview to display image attachments. Other attachments were processed separately but had similar workflows.

0 - https://www.fetchmail.info/fetchmail-man.html