But somewhere in the design of Outlook was an implicit 2Gb limit : The max size of a PST file. Maybe it was an OS limit, but still the decision to store all email in a single binary blob wasn't sufficiently forward-thinking.

I suspect that means a .PST is mmap()'d (or Windows equivalent), 32-bit Windows gives each process a 2G address space by default.

If that were the problem, the limit would be substantially less than 2G because of the address space needed for the DLLs, the stack, other allocated memory, and so on; and, of course, the space consumed by address space fragmentation.