He processes mail by stack: have his secretary print out emails and put them together with snail mail on a stack. Look at the stack every few months.

Urgent mail is expedited at his secretary's discretion, of course.

This requires a secretary (or at least an AI that's able to prioritize as well as a human). GMail is getting there by filtering out obvious cruft (spam filtering, flagging as important, moving social and promotional emails into their own tabs, etc), but falls down in terms of prioritizing non-obvious cruft.

Probably this: http://www-cs-faculty.stanford.edu/~uno/email.html

Interesting read, thank you!