Tracing an anonymous letter through a commercial courier service was, in theory, impossible.
In practice, it was merely very difficult, which was not the same thing.
The difficulty lay in the structure of commercial courier systems: they collected letters at distribution points throughout the city, moved them through sorting facilities, and delivered them without maintaining records that connected a sender to a recipient. The design was not meant to protect anonymity specifically; it was simply that recording sender information was expensive and provided no commercial value to the courier firm.
What the system did record, because it was necessary for billing and dispute resolution, was the point of acceptance — the specific distribution location where a letter had been submitted — and the time of submission. These records were kept for thirty days before disposal.
Kaien, when I explained what I needed, left without further questions.
