The post office on the factory grounds is always busy, queues of people milling about it's entrance. Inside are some clerks behind a counter with iron bars. The clients hand them their letters, which the clerks put into glass cilinders, and promptly stuff these into a myriad of copper tubes behind them. "POP"! Only, an underground network of copper tubes deliviring mail is nice on main land, but useless on an island.
Below the ground the letters wander around in confusion, and eventually end up at the post office.
Sometimes, letters emerge from the copper tubes. These, the clerks pick up with a sigh, and hand them to the post officers, which deliver them to an outbound ship or to one of the city's inhabitants, by pigeon.
A big envelope, uncollected by anyone for years, lies atop a shelf dusting away.