More interesting if you could make a mechanical one that would remove one envelope and forward the letter.
Then each data facility could reasonably process a few thousand letters a day, batching them for the postal service. After a few rounds of mixing, if there was significant facility-to-facility traffic, it would become impractical to find any specific letter's path.
Time for real world implementations of our high-latency packet routing algorithms?
The next step is building such a system for personal transportation pods, so nobody really knows where you're traveling ;-). However, you'll have to pack enough food to be shuffled across the country several times on underground pneumatic tubes as your personal carrier onion is unwrapped and retransmitted.
Then each data facility could reasonably process a few thousand letters a day, batching them for the postal service. After a few rounds of mixing, if there was significant facility-to-facility traffic, it would become impractical to find any specific letter's path.
Time for real world implementations of our high-latency packet routing algorithms?